What is the best type of underlay for vinyl flooring?
One way to work out the plywood shape is to make up a template out of heavy paper. You can open paper grocery bags out flat  for this and tape pieces together to make up the template. This can then be transferred to the plywood for making the cut lines. 

Two things to consider. 

Pickout where an edge seam will be for the plywood as it may very well be necessary to cut several pieces of plywood to get it possible to lay it into place.

Another thing is to remove any base boards and the toilet stool before laying in the plywood and vinyl. The vinyl is ideally placed under these items. Additionally the baseboard being re-installed means that it will cover where the plywood and vinyl do not fit perfectly against the walls.