VMRXX is really similar to VUSXX. I believe that there might be some small state tax advantage to VUSXX, but that likely doesn't have an affect if the money in question is in a 529 plan.
If you are looking for shelter from volatility you could move money out of the equities index funds that you are currently invested in into VMRXX.
Worth mentioning that just like VUSXX, there could be a theoretical loss of value in VMRXX if the government defaults.
I truly don't know of a safe haven in that scenario. Hopefully it doesn't happen.