A vix option does not price off the index you see quoted. It prices off the volatility-futures contract expiring at the same time, because that future is what a dealer can actually hedge with. Spot VIX is a calculation, not a tradable asset.
This is why a VIX call can barely move on a day when spot VIX jumps. If the spike is expected to fade, the relevant future may hardly budge, and the option follows the future.
Example: spot VIX rises from 14 to 18, up 29%. The relevant future rises from 17.5 to 18.5, up 6%. A $20 call that you bought expecting the 29% move gains only what a 6% underlying move justifies. Always quote the future before quoting the option.
Related: vix, volatility-futures, vix-futures-curve, vvix