Okay, because the VTR has the ring at the top of the tank area, your choices are going to be limited, unless you are willing to modify the VTR and cut off the metal band. That said, I would recommend that you look at the Kanger AeroTank (not the AeroTank Mega). It is supposed to be exceptional, it is easy to use, and it will fit in the ring on the VTR. If you don't mind using the extension tube and mounting the tank on top of the VTR, instead of down inside it, then almost anything is fair game. If you want good production from an easy to use tank, then go with the AeroTank Mega, or the Aspire Nautilus. If you don't mind building your own, then the Kayfun Lite Plus is supposed to be exceptional.
Personally, I like my Genny, the Z-Atty. It has by far the best performance of anything I have tried. I have NOT tried a KFL+ yet, so that might change at some point, when I do get a chance to try the KFL+. Everyone has their own different likes and dislikes, so you will probably get a ton of answers that are all different. It will be interesting to see what you end up with and how you like it.
Good luck, HTH and as always ... YMMV!