Background: I have been using Linux almost exclusively since 1999, and I have used it with Pianoteq occasionally.
I used it only occasionally because I did not have a dedicated music computer, and so setting up / tearing down computer and cables was too much of a pain and I often ended up utilizing the internal sounds of whatever instrument I had (this will change if/when I complete the Mybrid/Cybrid project). Even more rarely, I used a Mac for some experiments with other virtual instruments, DAW and Pianoteq itself. Without any doubts, I can say that the Pianoteq + Linux is the most reliable way to have something work out of the box! It worked all the times, with any hardware I threw at it (computer or instrument, MIDI-over-USB from the piano or MIDI-from-the-piano-to-cheapest-MIDI-to-USB-converter-from-amazon and more). Always fine at first shot! Admittedly I never had a VPC1, but I had many brands and models throughout the years. Everything else I tried (Reaper, Sfizz, Carla, Kontakt, Aria, and a few other things that I don't remember because I don't have that Mac anymore) either on Linux (for the ones that work on it, such as Reaper) and especially on the Mac have always caused problems and were not recognized easily.
All this long story just to say: if you are experiencing this problem, and you know for sure that you don't have a hardware problem, you have the wrong distro or the wrong version of that distro. I tried many distributions, and I can say without any doubt that the very best one is Ubuntu LTS (not the regular Ubuntu). If you are not strongly attached to that installation you have, I recommend you dump it (yes, I tried Mint myself and used if for a couple of years -- it was ok, but Ubuntu is better and if you haven't used it for a while, it has recently been improved even further). The next version of Ubuntu LTS is due to be released next week (and they are released only once every two years, so I suggest you wait for that to come out instead of installing a 2yo one). Meanwhile, if you have an old medium (CD, USB dongle) with an old Ubuntu (*), I suggest that you try that "live" (without installing on disk) and see if Pianoteq works correctly there.
I personally strongly prefer the "Mate" flavor of UbuntuLTS, but if you don't, the "straight" Gnome3 or KDE, or LXFC or whatever you prefer are equally good -- for those who don't know these are identical engine just with a different GUI interface.
If instead you do feel strongly attached to Mint for non-piano reasons, let me know, and I will be happy to help you debug your issue with it, but I suspect it will be longer than moving to Ubuntu (and you'll get a few other non-piano-related quirks too, which are the ones that made me grow tired of it and switch to Ubuntu). Since the "live distribution" test is an almost-zero effort, I suggest that you do this first regardless and then get back here with the result, even though your intention is to fix it in Mint.
Hope this helps!
Happy Easter to you and to all pianoclack addicts like me who cannot spare checking the site even on the very Holy Easter Sunday!!!
(*): or if you don't have bandwidth limitation download one