These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Ass Pull: Your Mileage May Vary, of course, but while it makes sense how sunlight and water could act as some sort of defibrillator, no explanation is ever given for why blood from resurrected vampires will cure vampires.
It's possible that The Virus was mutated by the accident, not eliminated by it. Viruses have been known to absorb one another's DNA, so if the live, unmutated virus came into contact with the mutated virus, instant cure.
The fact that it does seem so unlikely might at least account for why Elvis had never thought to try passing the cure along via his own blood prior to finding out accidentally with Frankie.
Nightmare Fuel: As karmic as Bromley's death was, the depiction of just how far gone the vampire starvation has progressed can certainly cause this.
The scene at the beginning of the movie, where a customer complaining about the lack of blood in his coffee turns into a riot.
Squick: At one point someone is clearly showing drinking coffee, with milk... and blood.
Not to mention how horribly wrong a small argument over how much blood was in the coffee went...
The coffee shop's blood bags are marked by type, suggesting their customers were actually offered a choice of flavors when supplies were still plentiful.