Loss on mutual fund shares held 6 months or less. If you sell shares of a mutual fund at a loss, and those shares have been held for 6 months or less, then there are special rules that may alter the loss you claim. First, if those shares produced any tax-exempt interest, then the loss is reduced, dollar for dollar, by that interest. Second, if those shares were held while the fund distributed (long term) capital gains, then the loss is treated as a long term loss up to the dollar amount of the distribution those shares produced.[1][2] Vanguard, and likely most other brokerages, will not make the correct adjustment on your IRS Form 1099-B, so you will have to correct it yourself by filing Form 8949 and changing the type of capital loss.[3]
Note that most Vanguard Tax Exempt funds (and others like them) are not subject to the preceding 6 month rule because of the way they accrue and pay out interest (dividends). The 6 month rule for tax exempt interest comes from 26 U.S. Code § 852(b)(4)(B), but there is an exception in 852(b)(4)(E) for funds that declare dividends daily and pay them monthly or more frequently.[4] [5]
However, ETFs and the Tax-Exempt Bond Index Fund (which is the mutual fund class of an ETF) are subject to the six-month rule, because they declare dividends monthly, not daily. To see whether a fund is exempt from the six-month rule, check its prospectus for the statement, "dividends are declared daily and paid monthly"; if it says "dividends are declared monthly" (or quarterly), the six-month rule applies.