It is generally assumed that when you relinquish control over the undead (which you can do, per Nox’s answer), they aren’t compelled to obey the last orders they were given, and therefore they don’t. Actually, it’s generally assumed they start terrorizing the populace and attempting to sate their particular hungers.
Strangely, however, nothing outright says this. Skimming the relevant sections of Libris Mortis, it’s stated that spawn gravitate towards their creators and will automatically return to servitude if a cleric disrupts that bond, but spawn are different from rebuked undead. No mention is made of necromancers’ control here.
That does, however, indicate the traditional approach here: delegated control. Through the use of the appropriate undead minions, one can effectively lead an army by controlling creatures who in turn control more creatures.
But since you are the DM, you could easily rule that this isn’t entirely necessary and that the player can try to use clever temporary applications of rebuke undead to effectively control more undead at once. It sounds like you find this interesting, which is the usual reason I wouldn’t recommend it: personally, I’d find it a headache to keep track of as a DM. But if you wouldn’t, then you certainly can do this. It does make the necromancer more powerful, but in a complicated and involved way, that should lead to more a more interesting game (unlike usual routes to extra power, which lead to a more boring game because it eliminates challenges).
They can, with a catch and a few assumptions.
The only way RAW I could find to resurrect the victims using the Resurrection spell is by first casting Greater Restoration in order to remove the effect of the vampire's bite (since the victims are not vampires yet), and then cast Resurrection. The Resurrection spell on its own would fail, since the bite is still in effect. This would need to be done within 10 days, as otherwise the effects of Gentle Repose would go away and the victims would rise as free-willed vampire spawn.
This however, assumes that the vampire's bite, or at least it's health reduction component is a magical curse, that Greater Restoration can be cast on a corpse that is soon to rise as an undead without intervention (on the same principles that sleeping creatures are nevertheless creatures, even though the victims in the case would be temporary corpses if left unattended), and that Greater Restoration covers vampirism (assumption made from lycanthropy being able to be cured by Greater Restoration).
Best Answer
Technically, yes.
DMG 260 states,
The lowest level creation of undead is the "Animate Dead" spell, which states:
Thus, if you lose control of it and then attack, it defends itself, and you have yourself a fight where you can plausibly gain XP.
It's not worth it
Assuming your DM is somehow ok with this ploy, killing a zombie or skeleton nets you 50 XP. The earliest level you could do this is 5th level, where you get two spell slots to cast this spell. Assuming that you're the only one fighting the zombie/skeleton, you get an extra 100 xp a day. You need an additional 7500 XP to level up from 5 to 6, so you'd get 1.3% of a new level each day by doing this ploy, or 77 days to level up. Note that I'm describing the fastest progression--this strategy gets even less effective at any higher level. Doing this also means you can't use your 3rd level spell slots for anything else.