ok i'm kind of new to vans but I know that every thing I'm going to say applies to all other hondas (civic accord prelude s2k integra etc) so it might give you a good place to start.
Do you have the ability to check low and high pressure on the ac lines? if so what are they and about what is the out side temp?
Have you checked the water drain for the evap? That can clog and sometimes fill with water causing the evap to freeze.
Is there a bunch for leaves and trash in the evap. box?
Those are the easy things to check.
If the evaporator box is clean and draining and you have the correct amount of refrigerant I would think that either the compressor is worn out/damaged which you would see with a gauge set hooked to it even a cheap low pressure gauge from advance auto parts would answer that (should be pretty high pressure with comp. off and drop to between 30 to 45 psi with compressor on).
It could also be the expansion valve but I personally haven't run into many of them going bad. Just try checking those things and get back to me with what you find out.