This one started out as in Inktober drawing in 2015:

Having them as actual vampires was a better joke, but I thought I could make up for it with cute bats and funny costumes.
For the nurse, I googled what vampire bats usually feed on and it turned up cows, horses and pigs. I'd drawn a cow already and a horse seemed like it would be hard to draw screaming, so I went with pig. It wasn't until my 6yo peeked over my shoulder and asked if it was a "Piggy Bank", that I made the connection! π