Link to the portafilter? Would be surprised if it mattered which basket you put in because as far as I'm aware the depth is the only difference between them (been years since I bought a regular-sized basket as I'm stuck with little 51mm baskets, so maybe things have changed recently).
The vst comes in either 15/18/20 grams. But the bottomless portafilter says 14 or 21g.
Am I right in think a 15g vst and 14 portafilter will work?