Ken, Basically there are 3 different styles of vias in the tool. The simple closed form , detailed closed form and 3D. Originally posted in cdnusers.org by horner
If you choose the detailed closed form option it actually runs the FSVia routine to get a pretty close result to the more computationally intensive 3D solver.
Now there is a variable "Set_3D_Via_Solver 1" (set this in the pcbenv\env file) that will force the fsviasolver executable to be run for all the extracted vias. This is much slower, and we have found to be not much more accurate then the detailed closed form results. This solver isn't officially supported right now however.
Refer to Solution 11098106 on Sourcelink for some more info.
Also, you can type in FSvia with no arguments at a command prompt to get additional information.