It doesn't really matter, there are no VIX underlying shares to be had so exercising VIX calls that are in the money is more or less equivalent to just selling them ... so just sell them, its even better then it would be to exercise them because unless it is day of expiry they will still have some time value to them and all their significant IV. The only things that matter in such a situation is liquidity and VIX has a very liquid options chain. If you want American style options you can use TVIX instead. But again it won't really matter, you are generally better off selling these options instead of exercising them anyway due to often significant time value and IV on them.