The Federal Reserve Board sets margin requirements via Reg T. For narrow based index options such as the VIX, it's:
- 100% of option proceeds plus 20% of underlying security/index value less out-of-the-money amount, if any, to a minimum for calls of option proceeds plus 10% of the underlying security/index value, and a minimum for puts of option proceeds plus 10% of the put’s exercise price.
In approximation, it's about 20% (brokers can require more). There's no way around that.
A covered call would make no sense because here because there's no underlying equity for the VIX and even if there was, equity margin is 50%.
There might be something that you could do to lower your margin with futures but that's above my pay grade. And, it completely alters your strategy.
Buying an ITM call also changes the entire dynamic, creating a vertical (or diagonal) with a completely different risk/reward. It doesn't make sense to me to alter a strategy completely for the sake of margin.
Buying a cheap OTM call might be an alternative but there's a trade off between cost and margin. The further OTM it is, the cheaper it is but the higher the margin (difference in strikes less credit received). You'd have to run the numbers to see if the margin is reduced enough by buying protection. Protection is good but not if you don't want it. Then, it would be tantamount to throwing away that long premium, well, at least until the VIX bites you.