Flaw in Vampires' Plan

After seeing this a few times, I've come to realize that there's a serious flaw in the Vampires' plan to permanently blot out the sun. If they do that, they're, eventually, going to kill off their food source -- humans!

Let's figure, the sun disappears, & all or most of life on Earth freezes &/or starves to death (including humans). So, what will the vamps do then?