VIX has been rising slowly over last 3 weeks and, more substantively, the VIX/VXV ratio has been rising too. As a result of that ratio rising, long VIX derivatives have been seeing lower contango rates.
What that all nets out to is that the market is anticipating more than the VIX itself tries to predict. This anticipation is higher than the March 3 through 17 period even though the VIX itself is lower right now. What does this mean? It means that the people who trade "what will the VIX do" are well ahead of those who trade the things (OTM options against SPX listed stocks) that make up the VIX .
That early March period created the range that the SPX then traded for about 12 weeks. One can assume that traders got comfortable scalping that range which, by sheer math, is something that will lower the VIX . We may be seeing the same "range setting" going on now. But keep in mind that there is no guarantee that a range will hold - thus the higher "tension" in the VIX that isn't reflected in the VIX itself.
How to trade it? You can range trade going long the VIX long when the S&P500 is at the top of range (like now) without too much fear of contango chewing your long position if held for 2 sessions or fewer. Shorting the VIX long when the S&P500 is at the bottom of the range won't get you the kind of contango tailwind that such a strategy normally provides.
Also, and this is important, the "tension" here is coming from your more sophisticated traders and algos - defy many things, like the Black Scholes equation. If they are betting on a big move, be prepared for this tight range to be blown out.