Yes. The legally of casting the spell is checked before you get to activate mana abilities and pay the costs. There is no more check for that legallity. So you have determined, that your spell casting proposal is legal, then you get to activate the Tower to get mana to pay for it, then you pay.
Yes, you can. Producing mana and paying costs is done in a later step of the casting process, after Feast of Blood has been placed on the stack and the check for you controlling two vampires has been made. From Gatherer:
Whether you control two or more Vampires is checked only as you try to move Feast of Blood to the stack as the first step of casting it. It doesn’t matter whether you still control two or more Vampires as you finish casting Feast of Blood (in case you somehow sacrifice one to produce mana) or as Feast of Blood resolves.
Private Mod Note
():
Rollback Post to RevisionRollBack
I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
Can I sac one vamp with Phyrexian Tower to play FOB ?
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)