Assuming that the PolygonIndex and PolygonPosition sets are in the same order, then given some polygon indices, you could get the corresponding polygon positions by using the indices to index into the PolygonPosition array.
Here, I’m using PolygonInsideNull, which is a set of polygon indices:

Here’s another screenshot that I took to verify that I was getting the right positions.
The purple numbers are the PolygonPosition and PolygonIndex arrays: