So I guess for the VPP this is not needed, as they only come in fixed sizes. (I understand this is much easier to model, but a flexible VPP cap would be something nice for the future.)

Yes, the VPP models just come with a total capacitance value. A "flexible VPP cap" is very unlikely. The purpose of the VPP cap is to cram as much metal surface area into a given area as possible. The resulting capacitor is then also crammed full of hard-to-model geometries. The simple "area * area-cap + perimeter * perimeter-cap" approximation will be way off, and the only way to really know what the capacitance is, is to fabricate it and measure it. I'm not sure that even the complicated parasitic model used in the Calibre PEX deck will get it right, although it would be interesting to find out.

Not done in SKY130, but in other technologies Calibre PEX does get it pretty right, for even better modelling EMX is a good choice, this is accurate to >100GHz.