Vanguard isn't in the habit of waiving fees temporarily to attract new investors like many of its competitors do. However, in recent years, expense ratio limitations — don't call them "waivers" — for its money-market funds seem to have emerged out of nowhere, with little explanation or fanfare. And now that interest rates have gone up, they've returned to nowhere.

In 2021, when interest rates were almost zero, it cost Vanguard more to operate funds such as Vanguard Municipal Money Market, Vanguard Federal Money Market Fund, and Vanguard Cash Reserves Federal Money Market than the income generated by the debt the funds were invested in. So, in order to avoid the funds having negative yields or reducing their net asset values below the promised $1 a share — what is known in the money fund world as "breaking the buck" — Vanguard as well as many other fund companies in the same boat quietly absorbed the costs of managing them.