You're welcome.
I'm not too familiar with the FH. Does it allow separate FX send and FX return blocks or is there only an FX loop block?
If it allows separate blocks, I'd put the FX send before the FH amp and the FX return after it. That way it totally bypasses your FH amp when using your Vetta preamp.
If it only allows a FX loop block, I think you would want the FX loop in front of the amp block, and disable the amp block when the FX loop is engaged, because otherwise you'll be running through your Vetta preamp and then back into your FH and through it's amp as well.
You might look on CustomTone and see if anyone has posted a 4 cable method patch or template.
And I wouldn't worry about the stereo cables if it's working.