Quote from clarodina:
don't know why vix option should use futures vix cos the cash vix price determine whether the option is in at or out of money. If a out of money option is brought where the strike price is 32.5 and the cash vix rally to 35 while the futures vix rally to 32, the option is consider in the money (using vix cash). If using the futures vix pric, the option is out of money. So why uses futures vix to trade vix option? For implied volatility?
VIX options are exercisable only at expiration. And you cannot buy or sell the VIX cash index. So prior to expiration, the VIX cash index has no practical, usable, tradable, arbitrageable relationship to VIX options.
The VIX futures converge with the cash at expiration. Prior to expiration - unlike the cash VIX - you CAN buy and sell it. Unlike the cash VIX, it IS a tradable instrument that can be used in every way as the underlying for the same-month VIX options.
Imagine for example that the VIX June 30 puts are trading at 3, the VIX June 30 calls are trading at 3, and the VIX cash is trading at 25. If the VIX cash were the true underlying, there would be a way to lock in a 5-point profit. But in fact, there is no way to do so.
However, if the VIX June 30 puts are trading at 3, the VIX June 30 calls are trading at 3, and the VIX June futures contract is trading at 25, you can buy the futures at 25, buy the puts at 3, and sell the calls at 3 at a 1-to-1-to-1 ratio, and you have a locked-in profit of 5 points each (not taking into consideration cost of carry).
So you are making a very common but foolish mistake if you are in any way conceptualizing the VIX cash as the underlying for the options, rather than the futures.