others are under the misinformed opinion that to be able to do something instead of moving you have to be able to move. break down of the logic of the rules:
if you are outside the garrison, < 6" away and all enemies are >3" away, then you can garrison instead of move.
if you are on the BVW you cant move. the wizard and the BVW are treated as a single model.
when you garrison you remove the unit from the table and treat it as if it were inside the garrison, and measure all distances and LOS from the base of the Garrison.
thanks to the skitter leap FAQ that applies universally, there is no argument as to wether or not we can LoSaT on a BVW and return to the table still on the BVW. as the whol ething is one model and returns in that unified state.
thus, you can garrison while on the BVW.
if you are already in the garrison, rules state yo can attack cast spells and use abilities as normal. so that's even easier.