I am unable to use voice on vatsim at the moment because I keep getting mic feedback with the FO's radios on the same freq. I tried turning off his radios, switching frequencies, changing the com channel, etc. So far none of that has worked. What am I doing wrong here?

That's why we have "soft mute".  Or if you prefer, Hard Mute.

Assign whatever key you're using for your PTT key and assign that to the Soft Mute key.

This way when you talk to ATC, the FO won't start acting on anything you say.

 

Best,