I would grind the vibram down on the non tread side before you glue it on. ISO standard is 20mm for the toe and 30 for the heel. If you put rocker on, you will have to play with those numbers a bit. Just think vibram + plastic should equal roughly those figures.
If you grind after you glue, you are losing traction from the tread which is one the big reasons your doing it...at least that's why I've done it in the past.