It was possible in SC1 but I can't figure it out for SC2. Basically, I want a trigger that when activated, disables vision for the player so they can't see their units and must depend on another player's vision. Is such a trigger possible?

I would suggest simply giving the units to a different player (player doesn't even need to exist I think).

Wow... so they didn't include a vision property for units...
For now you could either make a debuff that lowers their vision to 0 and apply it
or, if you don't want the debuff icon appearing in the unit info window...
You could write a custom script... there's a property "Sight Radius" so I'd assume the property has a similar name if you can find it.

You can change a units sight radius to 0 but the player will still be able to see the unit. The unit will not be able to see anything.

thanks guys