can you cast wither bones when your own undead are in melee ?

[Endoperez]

Some ideas...
Staff of Storms grounds the vampires, so you can cast Wither bones once or twice safely. You probably won't have air magic though.

Banefire Archers could work, but would be expensive chaff if the vampires get to them. Risky.

If the commanders of that army are not undead you could try casting something that has no effect on undead. Quess who will be targeted...