I've not played AIW via vassal as yet, but I have played PL. In PL you play by trust. When I'm watching my opponent's move, if I see something I want to Op Fire at, I immediately interrupt his file and resolve my Op Fire, returning the file to him showing the result. The rest of his move file is ignored.
Thanks for responding to my question! My opponent and I will give your method a try and see what happens.
All the best,