I find vampire gnomes show up easily so I am not 100% sure of what makes them appear, however, the thing l believe about Vampire gnomes is that they came with Late Night (I think) and I think you need about 6 plasma fruit plants of outstanding quality or better to get one or more vampire gnomes. I always end up with big gardens in my games and plant play,ma fruit to get these gnomes.
Once you have 1 vampire gnome you can get more by buying teddy bears and placing them where your vampire gnome can find them. I have done this and seen the vampire gnome turn teddy bears into vampire gnomes. Since teddy bears are quite inexpensive and vampire gnomes are worth more. Some people allow their vampire gnomes to turn teddy bears so they can sell them. It can be a good easy way to get simoleans.