Ok ladz, I've started by own orc army and have decided to make it interchangeable along with undead. Well that's the plan at least and I decided to help ya out wiff dis project.
Alright this should be a cool list:
Black Shaman Lord
Black Shaman (corrupted shaman who studies necromancer arts)
Servants (regular skeletons of dead foes)
Dead Unz (sorta like big uns but better)
Blood Boyz (beserk orcs who feed on blood, sorta like strigoi)
Bat Boyz (greenskins that ride bats)
Horror (basically undead giant but a billion times scarrier)
Spider Spawns (spiders)
Green Ghost: Ghost of the dead waaagh or something like that.
RULES: I propose that vampires choose special traits and such. Like they can be either orcs who would be killin machines but slow. Or goblins who are fast and lightly armored and a bit better at magic.
I'm about to head off on a looooooooooooong drive towards gaming, but I'll pop a link to my old diary here later. It's basically what you proposed :P
That's only on the modelling side though. Given the current ruleset it'll be a right bugger to integrate the two armies... but we'll try!