Player's Handbook II (3.5) / Divine Conversion / p193
This is a sidebar at the bottom of the referenced page, which is part of a section on rules for retraining. If you don't have the book, here is the direct quote:
DIVINE CONVERSION
As noted in the Player’s Handbook, a cleric who grossly violates the
code of conduct imposed by his deity loses all spells and class
features and cannot attain any more levels as a cleric of that deity.
All these penalties remain in effect until he atones. But what if he
doesn’t want to atone? What if a cleric of Hextor finds new meaning
and purpose in serving Heironeous after a dramatic conversion
experience? Such a character need not become a multiclass ex-cleric of
Hextor/cleric of Heironeous. Instead, Heironeous can simply reinstate
the character’s cleric powers once he has proven his loyalty, talent,
and ability.
A cleric who changes his patron deity must complete a quest to prove
his devotion to his new patron. The nature of the quest depends on the
deity, and it always clearly reflects the deity’s alignment as well as
his or her goals and beliefs. To start the process, the cleric must
voluntarily accept a geas/quest spell cast by a higher-level cleric
of his new deity. During the quest, the cleric has no access to spells
or cleric class features—except his weapon and armor proficiencies,
which he does not forfeit.
Upon completing the quest, the cleric receives the benefit of an
atonement spell from a cleric of the new deity. The character then becomes a cleric of the new deity and is inducted into the clergy
during an appropriate ceremony of the DM’s choosing. After selecting
two of the new deity’s domains in lieu of his old ones, the character
has all the powers and abilities of his previous cleric level, plus
the granted powers of his new domains.
This method is the only one by which a cleric can change his deity.
The retraining rules can’t be used to accomplish this task—it is
simply too substantial a change in the character’s identity (not to
mention his source of power) to chalk up to a bit of practice in his
off hours.
This answer essentially reflects the ruling already found in the Forgotten Realms Campaign Setting, but it may be useful to know it's also located in a setting-independent rulebook, just in case there are any sticklers out there.
“Always” alignment does not actually mean always
Always: The creature is born with the indicated alignment. The creature may have a hereditary predisposition to the alignment or come from a plane that predetermines it. It is possible for individuals to change alignment, but such individuals are either unique or rare exceptions.
Note that creatures with acquired templates do not experience “birth,” so the first line does not apply. Becoming a vampire “always” changes one’s alignment, but in rare cases (perhaps as in the case of your NPC), that can be subverted.
And even in the cases of Evil vampires, they can be reformed. There’s even an explicit spell for doing that (santify the wicked from Book of Exalted Deeds), though I strongly encourage you to completely ignore it as it’s very poorly designed (like most of that book), and has some extremely unfortunate implications (if you ignore the fact that the books says it’s good, and read it, it sounds like a pretty awful, evil thing to do to a person).
Thus, yes, you can have a non-Evil vampire. Redeeming Evil creatures is not just a Good act, it is the quintessential Good act.
That said, no character is ever required to always act for the maximum Good; it is not an Evil act to choose to not perform a possible Good action.
That said, assault and murder are pretty much definitively Evil...
Evil Alignment is consistently not an acceptable reason to attack someone
Attacking someone without specific cause is assault, which is Evil and in most jurisdictions illegal. Continuing that assault until the target dies is murder, which is definitely Evil and illegal most everywhere.
A paladin who attacks someone purely on the basis of pinging for detect evil should, under the rules, fall on the spot, for willingly commiting an Evil act.1
This is described in multiple rulebooks. It’s one of the few things about alignment that actually is somewhat consistent.
Alignment is not a detailed or consistent system
Alignment is described in different ways in different books, and the definitions are vague, ambiguous, and conflicting. The system is a historical artifact of D&D’s roots: it is designed for a simplistic, hack-and-slash dungeon crawl, where the players are Good because they are the players, the goblins, orcs, and vampires are Evil because they’re the enemies, and no one ever thinks too hard about that. Unfortunately, D&D has evolved but alignment hasn’t evolved with it; though people play far more serious and varied games than a straight dungeon crawl, alignment is still the same nine boxes. Don’t expect much from it; I actually strongly encourage you to ignore it. Outside of those simple dungeon crawls, it causes more headaches and arguments than it will ever be worth.
1 I cannot more strongly recommend against the actual falling rules, however. Instead of stripping the paladin of class features (boring, interrupts the story, punishes the player), I strongly recommend switching the character to the appropriate alternate alignment variant paladin, so he keeps his powers they just become “dark” (or chaotic if that’s the way he falls).
Best Answer
Magically hiding your alignment is pretty straightforward. The 2nd level cleric spell Undetectable Alignment makes you immune to magical alignment detection. This would be susceptible to Dispel Magic, so you might want to invest in a magic item that generates a similar effect. An item that constantly protects you with Undetectable Alignment would be 6000 gp.
For hiding your evil actions from the party, take a look at this question: How do I Raise the Dead and Look Good Doing it? That question talks mainly about how to hide your use of undead and necromancy from other people, and has some pretty good stuff in it.
Based on your description of the character, I don't see any reason for him to be doing evil acts outside of using undead. Remember that alignment isn't a straitjacket. If your character's goals before were to help the innocent and save the world, that won't necessarily change now that he's trying to use undeath to protect his life. He's less likely to risk his life to save people, but that's hardly an evil act.
If you find that you do need to do evil acts for some reason, the best way to hide your actions is to do two things.
Give a reasonable reason to be away from the party for a while. You certainly don't want to be sacrificing children in front of the paladin, so you need an excuse to be away from the party. Take a magic item crafting feat, and build a small lab/shrine in an enclosed space, away from the party. That way, if you need to do something bad that takes time, you have a good reason to hang out away from everyone else. You'll want to make sure that anything overtly evil in your lab is hidden when you're gone (preferably behind a wall and past a sheet of lead), but that shouldn't be a problem.
When you're off doing evil things, act cool when you get back. If you spend a few days sacrificing children to your dark god while making magic items for the group, act like nothing is wrong when you get back. Party members generally trust eachother unless given a reason not to. If you say "I spend a few days making those magic items, and here they are", then there's not really an in-game reason for your party to distrust you. If they never look for evidence of wrongdoing, then they're a lot less likely to find it.
One thing to keep in mind is that your party will probably need a very solid split between player knowledge and character knowledge. It is nearly impossible to not make your fellow players suspicious about your evil actions if they're a secret out of game as well as in game. Nothing makes a player want to investigate more than passing secret notes to your GM. You're probably going to want to have a conversation about this with your GM and other players, just to make sure that when people try to find you out, they're doing it based on in-game reasons and not metagame ones.