(Chicago, Illinois, USA)
Are vampires naturally evil/demonic? What if you still praise god when you are a vampire, can crosses still effect you?
While vampires have a reputation for being evil, this is not necessarily true for all those transformed into vampires. Humans are all individuals, capable of both good and evil, so it stands to reason that a human wouldn't lose this capability when transforming from one "being" to another.
Of course humans see vampires as evil because we are their primary prey. I'm sure if they could think at our level, gazelles would probably think that lions were evil as well. Also, human beings as a race have a very real fear of being knocked off of the top of the food chain. If an alien race were to come to earth and take over, I'm quite sure we would see them as "evil" as well.
That said, vampires are sometimes categorized along with demons or other dark spirits as they are believed to be without a soul. Choosing to become a vampire is something of a deal with the Devil - an exchange of one's soul for power and immortality. Certainly there are consequences of this action, and the loss of one's soul is not something that modern science has any ability to measure the effects of.
Most importantly, we must consider the evidence from historical and mythological accounts of vampires. These accounts provide heavily weighted views that vampires are indeed of an evil and/or demonic nature. Modern academics and scientists often dismiss these eyewitness accounts without any further investigation, but that doesn't mean that they should be discounted. Basing an opinion on general theory rather than scientific observation and testing is not only foolish, but extremely unscientific.
Still, I have to believe that anyone - human or vampire - is naturally capable of controlling their own thoughts and actions. While someone may be predisposed to a certain trait or behavior, we are all capable of choosing our actions to a very large extent. Therefore, I believe that a vampire, while they may be predisposed to what we consider an "evil" nature, is capable of choosing their way of life, whatever that may include.
This personal choice can also mean choosing whatever religious or spiritual beliefs they want. A cross is only a powerful deterrent to vampires if it truly symbolizes something powerful to the owner and to the vampire. It is entirely possible that any religious symbol could become a positive symbol for any being that truly and deeply considered it a part of their salvation - vampires included.
For further information on vampires being historically considered evil, read the Q&A for Are Vampires Heartless?, and for more information on the effects of crosses and other totems, read The Power of the Cross.