It works either way. Pressurizing it is more efficient.
If you have large voids or air pockets, negative pressure does not have the same effect as pressure which pushes eveything through the system and catches the air in the expansion tank.
In addition, vacuum is not going to test all of the joints and hoses. Pressure will.
You can probably rent a Stant kit at auto stores that rent tools. Try one. You will like it.
To me it is worth the $120 or so to buy one. You no longer will have to hang the car from the rear bumper from a tree to get the air out...most of the air out.
Wait til you hear the car belching. It's like a volcanic irruption.