The headset adapter on the VTX works just that way. You use the PTT on the radio with your headset mic.
I've got two of the SPO intercoms. The reason that it has a separate PTT jack is because many of the PTT switches MUTE the mic audio when you're not pushing the button. Obviously, this means your VOX intercom isn't going to work. So rather than running the mic plug from the headset into the PTT and then into the intercom, they give you a separate place to plug in the mic plug.
Jim's RST intercom does it a different way. They just tell you to put the PTT after the intercom and IIRC they run two mic outs from the intercom, just incase you have two mic inputs on the panel (and separate ptt switches built in).