Why is a Werewolf not immune to Sanguinare Vampiris

There was a great walkthrough on how to cure Vampirism on Gamefront: due to that site's redesign, it's been lost, but here's the list, distilled:

• If it's within three days of becoming a vampire, you can cure the disease with a Cure Disease Potion.
• Activate any Shrine. The Shrine of Talos outside Whiterun will do. Shrines will cure you of all diseases.

Since it sounds like you're a Stage 4 vampire, you should feed so that you're no longer stage 4. To do this:

wait until night and drink the blood of a sleeping person. Even sleeping beggars will do. Just crouch/sneak and interact with a sleeping NPC to drink their blood.

NB: If you’re still having trouble drinking blood, remember that you have two useful abilities as a Stage 4 vampires — Embrace of Shadows, and Vampire Seduction. Embrace of Shadows turns you invisible, while Vampire Seduction will make an NPC calm and cease wanting to attack you.

Once you have contracted vampirism (e.g., gone beyond Day 4), you need to do the following:

• Speak with any Inn Keeper or Bartender about rumors. They’ll tell you that a man named Falion in Morthal is studying vampires.

• Find Morthal east of Fort Snowhawk. Once there, talk to Falion and he’ll discuss how to cure vampirism.

• He’ll tell you to get a Black Soul Gem. If you don’t have one, Falion will sell you one. Before being cured, you’ll need to fill the Black Soul Gem.

To fill a Black Soul Gem, you’ll need to cast a Soul Trap spell on a human, then kill them. Any human will work, including bandits and enemies.

• You’ll need to obtain a Soul Trap spell, sold by any magic vendor.

If you’re really desperate, try the College of Winterhold, or explore Whiterun to find a shop. If you don’t have high magic skills, just purchase a Soul Trap scroll. NOTE: The Mage for the Jarl of Whiterun, the Jarl’s Court Mage, should have a Soul Trap spell.

With a filled Black Soul Gem, return to Falion and talk to him to complete the ritual.

Yes, you can still become a vampire. Health potions (potions of Healing or Health) can't cure diseases. (The 'Cure Disease' potion can, however.)

You should still have Sanguinare Vampiris even if you used a health potion, and will eventually progress to being a Vampire, provided that you don't cure your Sanguinare Vampiris disease.

Note that:

Whenever a vampire casts Vampiric Drain on you, there is a 10% chance for you to be infected with Sanguinare Vampiris.
...
However, rather than letting them hit you in melee, you should keep your distance and only allow them to cast their draining spell as this is the only attack that can impart the disease. To speed up contraction, repeatedly lose and gain sight of the target vampire so the spell is cast multiple times. The check for giving you the disease is passed only when the draining spell hits you, not over sustained action.

^{From: http://www.uesp.net/wiki/Skyrim%3aVampirism#Becoming_a_Vampire}
If you have already contracted Vampirism (gone beyond the third day of infection of Sanguinare Vampiris), you'll need to talk to Falion and give him a filled Black Soul Gem to cure yourself of Vampirism.