Send (VH4) to in (left mono on G major) and Left Out (G major) to Return (VH4). Think of it as sending a preamp signal to the FX unit and returning the processed signal back to the amp. But, you have choices....
Send 2 gives you the option of switching the unit out of the circuit. Send 1 you cannot switch out. Use either, depending on your goals.
Return gives you some options which will affect your tone and all the A/D/A conversion that goes on with multi-FX units. The parallel return will let you use the mix volume control on the front of the amp to determine how much of the wet signal is blended with the dry signal and will at least let you retain some of the $4500 unprocessed Diezel growl. The serial return is all converted A/D/A and will definitely make a noticed change in the overall tone.
Depends on what you are after.
Steve