First, don't park it in actual cash or you'll lose value to inflation which is currently running high. At a minimum put it in treasuries.

Second, trying to time the market is almost always a suboptimal strategy. The question is when will you likely need the money? If you won't need it for 10 years or more, keep it in index funds. Otherwise, treasuries.

Personally I tend to assume anyone talking about VTI knows about the risk-free rate.