In my vanguard roth IRAs brokerage account, uninvested/excess cash is kept in a money market fund, VMMXX. I dont care about earning return on this money; I just keep it in cash to help with rebalancing and in case I want to buy a dip/recession. The expense ratio is 0.17% so doesnt that mean that for every 10K I keep in the money market fund I lose $17/year? Should I care about this? If at the start of the year, the funds NAV is $1, and at the end of the year, the funds NAV is still $1, I havent lost anything right? (apart from inflation)
Not really. The expense ratio is taken before distributions are paid which applies to all mutual funds.
In this case not really. If it was a taxable account, then other options may be more tax-efficient that is worth noting. The key with money market funds is that the expense ratio often represents how much money the administrators will take before paying out the rest. So, if your money market fund bought investments that paid .25% then you'd likely see .08% as that is what is left over after the .17% is taken in the dividends.
Wikipedia has a good article on money market funds. Keep in mind that most money market funds are run as one of a number of funds from a fund family that may have to take a little less profit on the money market funds when rates are low.