How to find current limit or resistance of current path that is a polygon in PCB?
I would calculate the resistance due to the thermal reliefs, and the resistance due to the vias (if you want to include the vias in the path under consideration). The resistance due to the polygon will be negligible compared to these resistances.

If you want to figure out the contribution of the polygon anyway, first figure out the surface resistance of your copper, in ohms per square (You'll find that the resistance between opposite edges of a square of copper sheet will be equal, regardless of the size of the square). Then break up the polygon roughly into squares between the two vias. Then figure out the resistance between the two vias in terms of series and parallel combinations of copper squares.

Or, if you want a more accurate result, you'll need to resort to a finite elements model. This can also account for "spreading resistance" around the vias.