why doesn't this work. Create a vespen tank at a 1x1 location.?
As i recall, X-tra powerups cannot be created by triggers, only preplaced and then moved.
No X-tra units can be made at all. I think there are only a few exceptions. (Unless those units are not considered X-tra units)
If you only need to create a limeted number, you could put like 40 of them stacked at some unused spot on the map and then move them one by one when you need to "create" one.
You can't stack powerups.
Just do this, it creates the same effect.
At a corner or the map or an area that won't be used, Pre-Place with your editor as many Vespene Tanks that you are going to use.
Now when you need to create a Vesepene tank, just move one of the ones that you already Pre-Placed.
It also helps to reuse them and move them back, instead of killing them.