VXX options are not index options. They function like equity options. The underlying is not an index, but rather an exchange traded note that is tied to VIX futures. That's way too kinky for me LOL
VXX options have American style exercise, with a very real possibility of early assignment. And they require delivery of the underlying ETN.
I don't want to mess with those variables LOL
If you sell a VXX credit spread, and at expiration, the short leg is in the money and the long leg is out of the money, you have to buy in the short leg, or you will get assigned and be short the ETN overnight with no hedge.
VIX is cash settled, with European expiration. No underlying security to deliver, and no risk of early assignment.
Much cleaner.