Is vitamin D deadly to vampires?

Due to a discussion about why people feel cruddy when a day is overcast, I got to thinking is it really the UV light from the sun that kills a vampire? We all know that UV light is a source of vitamin D, which is a source of energy for humans, having an overcast sky blocks the UV rays, preventing the intake of vitamin D. However, this same light is what kills a vampire, yet, in many movies/books, vampires are able to move around during the day time if there is cloud cover or the sky is overcast. Bram Stroker's Dracula was able to operate during the day. If I remember correctly(it's been a while since I read the books) in the Chronicles of Lestat books and Interview with the Vampire series of books, Louie and Lestat were able to move around during the day time if there was cloud cover. The same goes in the movie Van Helsing, they could move about hinderless as long as there was cloud cover. Even in the Twilight series(someone shoot me for even referencing this) the vampires wouldn't sparkle if there was cloud cover.

So, with that said. Along with UV light sources and other deadly objects to them. Is vitamin D deadly to a vampire? All of the natural sources of it are deadly to the vampire, so if I were to sneak a multi-vitamin to a vampire and convince him/her to take it. Would said vampire burst into flames as if he/she were standing in sunlight?