In VT, we spar a knife man, with pen, against an unarmed fighter - all MMA rules of sparring apply, but the unarmed man has the added dimension of having to make sure he doesn't get cut - that includes in closing, in the clinch, and on the ground.
I always wear my best gi for this - which gives me the added incentive of not getting pen marks all over it!
We can also spar knife vs knife the same as we do stick v stick VT.
When WE "mix the martial arts" at STMA, we mix the weapons in as well!