I use a Barge Concepts VFB-X
http://www.bargeconcepts.com/x.html
If you look at the picture, you will see that it has four phono sockets - "In", "Send", "Receive", "Out".
If you are using the VFB-X with a mono output guitar, you:
. connect the guitar into "In"
. use "Send" and "Receive" as the start and end of your FX loop
. connect from the "Out" socket to your amp.
The "Clean Blend" knob controls the mix of dry and FX loop going to the amp - or you can do this using an expression pedal (purchased separately) plugged into the VFB-X.
To use the VFB-X with a 345, you need a Y-cable or a breakout box. You then connect the neck pickup into "In" and the bridge pickup into "Receive", and connect the VFB-X to your amp via the "Out" socket. This allows you to switch between the two pickups in the normal way and have a mono output.
More interestingly, you can leave both pickups switched on and pan between them using the expression pedal.
Even more interestingly, if I am using effects, say with my bridge pickup, I place these between the guitar and the VFB-X and use the expression pedal to blend in just the right amount.
RN
PS Unfortunately, running a 345 into the two sides of a two-channel Fender amp such as a BF or SF Twin, Deluxe or Vibrolux doesn't work. The output signals from the two channels are designed to be 180 degrees out of phase and when both pickups are switched on, the tone is thin and disappointing.