You can make a giant scorpion join the party, and attack with poisonous stingers if you want. There's even a sprite for it! But no faceset sadly. You know, it's RPG Maker, you can make a giant scorpion the hero who's skilled with ice elemental gun techniques, if you really want!
As for Vampires...if you want basic draining, that's really easy. I love how easy it is to make HP Drain (and MP Drain!) skills in VXAce! You can use events so you can't enter peoples homes uninvited when the vampire character is in your party. You can make their attacks bypass Reflect. You can make them weak against Garlic, Wood, and Light elementals...
As for sprites...nothing in RTP, but there's a good sprite made by PinedaVX
here. (scroll down until you find the actorsets with 8 characters per image, 4th row, 1st column, click and go back to top, there you go!)
Also, a
portrait for that sprite, by Soruve. (scroll down, you'll see it)
I know you didn't actually ask for that, and...this isn't the forum section to ask for that, but I just felt like throwing that in.
ONE MORE THING, I think I've seen a script that lets you recruit monsters into your party, without having to make them actual actors, but...I'm not really sure...and, it's probably hard to make things work out, like party group switching, actor based events, and so on.