Mechjeb will not attempt to affect attitude control if there are no active reaction wheels present, failing to utilise either control surfaces or RCS thrusters, vectored thrust engines being the only exception that I know of.

MJ use the RCS if you activate them manualy. I did not see a change in .21 that require change for the RCS torque
Control Surface need a patch, one is waiting is in the pull requests.
Vectored engine works indeed.
You can check the code in VessState.cs if you want ( torqueAvailable calculation ) ( is calculation english ? )