I've got a vac-u-lock harness and this one:
Sedeaux Vibrating Harness
I had bought the vac-u-lock originally because I thought it would be easier to handle, but it isn't it. The second 2-strap harness is much more comfortable and stable to wear. The vibrating part isn't really necessary however. The thing I like best about it, is instead of buckles, there are clips. It makes it super easy to put on and adjust, and also to take off. I plean on buying a second one, because whereas the vac-u-lock does let you swap toys more easily, the o-ring on the 2-strap is a little bit much to fiddle with. I'd rather just have all the toys set up and ready to go and I can hop in an out and be locked and loaded in seconds.
The biggest hitch for you I think is the suction cup - that's going to add "volume" i guess is the best word when trying to attach to an o-ring harness. I was able to use the vac-u-lock dildo I bought with my o-ring harness, but that's because it had balls.
Also contrary to what the poster above said - you don't have to worry about stretching the o-ring over the balls - take the o-ring out of the harness first, roll it over the "stem" end, and then attach it to the harness once it's at the base.