Just so everyone reading this is on the same page, this is about more efficient calling of the type, not of instances.
For calling instances, the vectorcall offset can be set using a "__vectorcalloffset__" PyMemberDef (see the end of the docs section, and read the linked docs carefully -- vectorcall is fast but, compared to tp_call, relatively tricky to implement).

I haven't checked if implementing tp_vetorcall correctly is possible -- remember that it'll need to match what tp_call does, including any changes to it in future versions. The bar is higher than with static types.

See devguide and vectorcall_limited for how to add tests for this. (If it doesn't make it into 3.12 it'll need to be in a seprarte file.) The tests should show that it's possible to correctly use the feature with limited API, as well as cover any edge cases.

I'd like to second this feature request. I was just looking into using vector calls to accelerate object construction for nanobind and realized that it's not possible in the stable API because of the missing type slot.