I personally don't think it was stupid. Of course, it definitely sounds strange for anyone to dare do something like that....but then, I don't think Vittorio's judgment was entirely sound at that time, either. I mean, if your whole family had been massacred, and the heads of your little sister and brother have been cut off before your very eyes, and you have held them in your hands, would you stay completely sane after that? I don't think so. At least, I'm not at all sure that I would, if I were in Vittorio's place. Add everything else - the knowledge that this has been done by demonic beings, the absolutely nightmarish encounter with these beings, the spell of stark raving madness, the angel visions, then having to finish with the whole vampire coven. Even the very fact that he had to seek revenge - I do understand him, and I know I would have done the exact same thing in his place, too, but can you imagine what an impact all the rage, the very feeling of craving revenge, and then the act of it, the murdering of all those creatures with his own hands and the cuting off of their heads, could have on a 16-year-old's mind? They could be evil and demonic, and they had killed his family, but it was still murder all the same, and bloodshed and blind anger. Vittorio must have been emotionally exhausted by the time he was alone with Ursula, at best, and perhaps slightly cracked as well. No wonder he gave in.
Actually, I admire his strength. It seems remarkable that what he went through didn't break him down completely and send him into fully-blown madness....or turn him into something like Armand. He must have been exceptionally strong and endurant for that not to happen to him.
And besides, he was a devout Christian; and in those times, people had very special ideas about God and demons and salvation. Who knows, maybe he really did believe with his whole heart he would save her if he did what she told him? I don't think it would even look too unusual, given what a background he must have had regarding all these things. And it's such a sweet thought, that you could save a demonic being by taking all her blood into yourself, especially if it is someone who had saved your life and whom you love dearly. :) I think it's very touching that he believed it and let her give him the Gift.
On the whole, the way she made him is just so sweet....those visions she gave him, of the field with the irises blowing in the wind and him and her running through it, the way she asked him to take in all her demonic blood to deliver her soul, and to be strong because it will be hard and will hurt....I think she did it out of love for him, and out of gratitude both for sparing her (un)life and for setting her free and saving her from Florian and the rest of the coven. I cried when I read that part of the book....
Just my two cents' worth :)