1.Can I use 1 ammo token in assault to up roll dice one point? Or only to fire?
This really is as written in the rules -- when you take a shot. Assault is a melee attack, not a shot.
2.Can I assault any vehicle Or only transport vehicles?
If you just want to assault a vehicle (to hopefully destroy it), then you can as long as you can normally damage the vehicle. If the assaulting unit has an X versus that type of vehicle then you cannot assault the vehicle without equipment that will allow it.
3.Can I use supression fire on a panzer vehicle for example if usually I cant fire it?
Well, if you double an "X" bonus, it is still an "X" bonus and thus you cannot hit the heavy vehicle. At least barring equipment that will allow you to actually hit the target. If you manage to get any other value other than an "X", then you can attack to suppress.