Vampires are generally assumed to be relegated to the world of fiction. There are people who sustain themselves on the life force of others - often called "psychic vampirism" and there are individuals who consume the blood of human although the human body is not adapted to be sustained entirely by blood so even the most dedicated "vampires" would need some element of a normal diet as well.
This site http://www.vampirewebsite.net/ has a good level of information and a comprehensive question and answer section. Possibly you will get a fuller response there.
Answer - I am afraid that there is no such spell. Legend/myth/tradition has it that a person becomes a vampire as a result of interactive blood exchange with another vampire and to the best of my knowledge there is no other way to make the transition.
From the little bit of reading I've done in this area I believe the amount of blood necessary is a nominal amount and not the pints and pints implied by Hollywood films and the like...
When seeking out a vampire I would advise you to be conscious that a good many self proclaimed vampires will actually be delusional frauds. Just as I don't take my broom and pointy hat to work most vampires won't advertise the fact by getting the local dentist to sharpen their teeth. Be careful, no doubt there are serious genuine people who have adopted neo Vampirism as a chosen lifestyle but there will be many more who seek to use the glamour and the mystery associated to prey on young people wishing to develop an interest in what they do. Don't take any risks.
More information here: Vampirewebsite.net -How to get turned into a vampire
There are two practical possibilities that I can see. The first is that your lovemaking (lucky you!) is so intense that it is leading to a mild halluciogenic response which is causing your boyfriend to perceive you differently. If you think of the hormones generated during very passionate lovemaking it certainly isn't inconceivable that they could have a mind altering affect. This to my mind is the most likely explanation - if you are having sex fuelled by alcohol/drugs then it becomes even more likely...
The second practical possibility is that women actually do change their physical appearance during sex. Your pupils dilate, your skin becomes flushed, your features contort... These physical changes very possibly do mean you look different to your normal self at this time. To be fair to your boyfriend, if he's in the midst of generating all this passion he probably isn't concentrating too scientifically on your appearance. The fact that these changes only occur when his attention is distracted and during what is a powerfully emotional time for you both leas me to believe that in this situation the explanation is a simple one.