I'm trying to create vespene tanks, via a trigger, becuase I don't want to place them until i need them.
How would i go about doing this? Is it even possible?
My first conclusion is i can't create neutral units via triggers... Please tell me I'm wrong... so i can do this...
The only other way i can think to do this .. is to preplace them on the map, then move them when i need them...
I would rather create them tho, when i need them.. I'm not a fan of preplacing...
I searched the wiki for creating units via triggers, but couldn't find anything
None.

I do stuff and thingies... Try widening and reducing the number of small nooks and crannies to correct the problem.
You cannot create that special unit with triggers (rule: You cannot create what you cannot set in staredit.).
Like you said, you have to preplace that unit and move it whenever it is needed.
btw, you cannot create units for neutral directly (only for p1-p8).