Basically, spells are direction of feeling and desires, manipulating the energy of the practitioner and sending it to the universe to let our desires a chance to come true.
As one of the people who left a comment in this thread said, energy manipulation and your will won't let you change your physical body, physical appearance or any form of your current DNA and genes. This is something that energy manipulation, feelings, desires and the will of the universe will not solve for you. To become something you aren't physically means that your genes and DNA and genes. will totally change into other forms. Not talk about creatures that do not really exist in this realm.
So here is the answer for the question "how can you know it":
1. You can not change your body physically. Same about your genes and DNA.
2. do not forget that vampires are creatures that cannot be found in this realm, so your problem is not only dis-ability to change you DNA, but also the fact that such DNA and genes of the creature you are talking about can't be be found anywhere.
3. Perhaps you do not know how magic and magic spells really work, and what they are meant to let the practitioner to do.
So, please do not give wrong information and mislead newbies with such horrible information. Perhaps that's why people here come and instantly claim to be able to do things that are not possible. This is very hard to see and understand how awful the situation is. If you stop giving such spells out, and understand by yourself that it is not possible- the situation might be a bit better. I have explained to you personally why your spell can't work here, so please do not develop this to an argument, because this is pointless.