Something like VFMXX could drop based off the market rates of the short term assets it’s constantly investing in.

The two are somewhat correlated but not exactly.

If you want something guaranteed to get a specific rate for a period of time, you need a CD. That will come up a timeframe, early withdrawal penalties, and a fixed rate (meaning if market rates go up, you lose out on the upside unless it’s an andjustable rate CD which are out there these days. Also means if market rates go down, you still get the higher cd rate)

Hope that explanation makes sense