First of all the tool can only affect a maximum of 8192 vertices in a single 2D Texture. This is because the maximum Texture size a Texture can be for DirectX 11 is 8192 pixels in either the X or Y direction. The tool generates the data in the Texture using the following formula.
This is taken from the doc, this means any vertex baking done in UE is stuck to at best 2048 polycount on each character, correct me if I’m wrong.