Besides Spellcaster's (excellent) point, "strong as a vampire" is sort of vague. It's not like there's some measuring system we have for aural integrity or spellcasting intensity. And even if there were it isn't like vampires, or anyone else, are of some identical form and strength.
Try making specific goals you'd like to reach. What /specific/ feats of vampiric strength are you hoping to be able to accomplish?
If your speaking of physical strength, you would need to eat a lot of lean protein and exercise a lot. Make sure to breathe properly when exercising and lifting weights as oxygen is necessary to building muscle.
Vampires are just an idea that came to life. But not in real life, but in the world of imagination of the human being. During the ages it appeared to be more modern in real life. It started from Dracula, and it ended up on Twilight and the other science fiction movies on Hollywood.