I hope your not using the vice grips on the nut, it will round it off and leave you with less options. Second, you need more leverage. I don't know what kind of wrench experience you have (so this may sound stupid), a piece of pipe is the easiest to explain, just slip it over the end of 8mm.
A more common way to get more leverage and harder for me to explain is to use a second wrench as extension handle to the 8mm (if you wrench a bit or been around a shop this will be of no help). Place the box end of 8mm on the nut and take a second wrench. With the box end of the second wrench, place in into the open end of the 8mm on one of the forks while keeping both in almost striaght line. The orientation of the second wrench with be pertidicular to the first. You wil l be able to use this as pry on the first giving you longer leverage. This only works with box that has offset and works with only with one sides pointed up, so you might have to play with it to figure it out. If you not tired using more leverage, maybe someone handy with a camera can send a pic. A picture in this case is worth a thousand words.
Again, you may be more advance have already tried this. Never use vice grips, It will cause more problems then they solve.
Good luck
Dyslexic Bill in Alabama