I've been keeping my emergency savings in VMSXX (vanguard's Muni bond money market) for a while now since on a tax equivalent basis, the yield (for me, it depe

I've been keeping my emergency savings in VMSXX (vanguard's Muni bond money market) for a while now since on a tax equivalent basis, the yield (for me, it depends on your income and state) is roughly 2.5-2.6%. Maybe I'll switch to this. My question is though: how the heck are they doing it, and is it sustainable?