I haven't done this, so this is off the top of my head, but the Ren is set up just like an MBox (or any standard 2 channel audio interface).
You have to first set the MPC as the audio interface in the playback engine of Pro Tools.
Open the MPC software as a plugin on an instrument track and load your project. Route the output of that instrument track to the main output.
Create a mono audio track. Set it's input as the Rens input 1. Plug the mic into the left preamp on the back of the MPC. Turn the switch from line to mic and turn on phantom power if using a condenser. If you plug the mic into the right preamp, then you have to set your audio track input to Ren input 2.
In the playback engine of Pro Tools, set the buffer to no higher than 128. Anything higher will be bad for recording vocals due to latency.
Set the Direct Mon knob on the Ren to the middle while recording. This will tell the Ren to output both computer audio (MPC plugin/instrument track) and input signal (mic/live vocals) at the same time.
Record arm the audio track and record the vocals. Once vocals are laid, you can turn the Mon knob back to the right (or leave it in the middle) and crank up your hardware buffer to mix.
Hope this helps!
Yezzir