This is a bit tough and won't be portable, but its not impossible. First you need to understand what exactly a member function is compiled to, then you emulate that with your ASM function. Important things to know are how the name mangling works or not forgetting that VC passes an implicit this pointer to member functions.
I think your best bet is to write the function in C++, compile into debug code, open that up in a debugger to see what exactly VC expects the function to be called and copy that together with the calling convention in your ASM code.
Also check out the
Agner Fog's Manuals.