Trying to get VarAC set up recently with my IC-718, I ran into an issue getting it to PTT via the serial port. I run a DigiRig which provides an all in one solution for various rigs. It provides a sound card and serial port via one single USB port on the device. The device also support triggering PTT via CTS on the serial port. Omni-Rig also supports this, but I think the application needs to specifically code for this.
In WSJT-X, I can use Omni-Rig for rig control, and with PTT, I select DTR/CTS, but for the port I select CAT instead of the serial port of the rig. If I try this setup in VarAC, I only have an option for the serial port my rig is on, and if I try it I get Access Denied on that port (because it's already open for Omni-Rig).
The explanation of that drop down says that CAT is a special port for special interfaces, like Omni-Rig.
Since my rig doesn't have PTT support through CAT, using Omni-Rig for PTT doesn't work. In looking at the source code for WSJT-X, it seems the omni-rig transceiver type will trigger the DTR/CTS without trying to open the serial port again.
It would be nice if VarAC supported this. For now I can just use direct access through the serial port and it works fine.