There is a diagram by the output jack: tip is Aux send 1, ring is Aux send2 (sleeve is ground). You can use an actual insert cable (TRS on one end, splitting into two TS cables/plugs at the other end) to feed each input of the 400. You need to route the output(s) of the 400 back to VF channel input(s).
The inserts can only be used while actually recording. Follow the tagging/labeling on your insert cable (ione cable for each channel).
Paj
8^)