For starters, I happen to know the VP4 was very well thought through. I know you don't want to hear this but you're in a very small minority of folks running stereo amps. I don't think it'd just be a matter of adding another jack - that's be a very awkward implementation that would have some strange documentation and configuration. To me, the logical implementation would be a pair of stereo ins and outs. I'm sure it was considered.
As you are probably aware, the FM3 has the IO you need to do 5CM for mono pre and stereo post effects.
If nobody uses stereo rigs, why is the default mode for the VP4 itself a pair of stereo ins and outs?
Also, do you think having an additional Output jack (or you know what, let's just say a devoted Send/Return loop like
@Deadpool_25 mentioned because that would seamlessly allow purely stereo operation as well, so two jacks) would be
less awkward than asking the user to choose to either run in Stereo or 4CM mode, then asking them to read up on how to change modes, and then asking the user to always remember, per patch, what mode the VP4 is in?
Or do you think it would be less awkward to have enough Ins and Outs that... everything just works all the time with no extra setup necessary?
Again, it's not that I don't think the VP4 isn't great, it is. It's definitely a Best In Class product. But it does bother me that mono pre / stereo post effects aren't available when it seems like 95% of the infrastructure (software and hardware) required to unlock the "full capability" of the pedal is already right there, minus a jack or two.
edit - you know, I'm not just complaining for complaining's sake. I've been on the Fractal Train since 2008 and I'm talking about this stuff in good faith. Fractal is one of my favorite companies in the industry and has been for years. Because of that, I feel I owe the company my honest feedback. However, in the spirit of arguing in good faith, I will say that one potential hangup I could see with my own argument is that if and when an additional dedicated mono Loop was activated on the grid, the VP4 would have to be told which side of the stereo signal to send through the loop Send output, which would mean having to configure the Loop Send to forward the Left, Right, or L+R signal, so that would take some extra configuration, and extra configuration is what I was trying to avoid with my suggestions. Still, that seems like something that could be done by just selecting a "Loop" graphic on the grid, hitting Edit, and selecting which side to forward to the Send output jack. Still seems simpler than having to reconfigure the function of the entire pedal to either Stereo or 4CM though.
Ultimately, the product is what it is, and it's a great product. I hope the feedback is helpful though.
Until then the best we can do is pick the best unit for our particular needs and implement workarounds as needed. 
