Von chastein armies use more traditional style of warfare, only with undead soldiers, Giving them a trebuchet or somthing similar wuld be perfectly natural.
The tomb kings on the other hand use much more acane magic, so they shuld probably have a bonethrower or somthing like that..

That said, i wonder what units of the VC you are planning to add,
I suggest the folowing:

Core units:
Skeleton regiment,*
Zombie regiment*
Armoured Skeleton Halberdiers

Spesial units:
Grave Guard
Black Knights

Rare units:
Bashee**
Wright Band ***

*alreddy made
** I dont know if it is possioble to make those, but they wuld make for a fine High tech unit players culd build twowards.
***I am hoping you will find a way to include wrights, cause they are Unspeacably cool. (A minimal untis size of wrights maby?)

For command units i sugest
General -> Von Carstein Vampire
Captain -> Wright Lord