A vampire still have a soul (spirit) in 3.5 as far as I know so since Astral projection states:¸
If the cord is broken, you are killed, astrally and physically
Dying ''physically'' or having it's body destroyed only forces the vampire to go to mist form and go into it's resting place.
Being killed ''astrally'' just mean your new form ''died'' and you normally return to your physical body, if the silvery cord is severed then even your material body dies, but that does not affect the vampire. (I might be wrong here)
As far as I understand, it would be clever for a vampire to be even more careful and only go out in his astral form (wich is not exclusive to the Astral plane, even if you are transported there, you can come back with your astral form in the material plane if I remember correctly from a post I saw). His material body would be not too far from it's resting place, in an extremely secure area, so if it dies, it would just be in his coffin and regenerate. (a fortress with a lot of adamantium doors to break etc. before reaching one of it's many resting place in a demi-plane so good luck reaching it in 1 hour ;))