Exempt interest dividends are reported in Box 11 of Form 1099-DIV.
You will need the "Supplemental Information" package from your brokerage/investment management firm (Vanguard) to see what states paid those exempt interest dividends. This information is typically given as percentages (%).
Multiply the number you have in Box 11 by the percentages listed in the Supplemental Information to get the amount paid by each state. Many (most) of the results will be very small and can be ignored - only amounts paid by the taxpayer's home state are usually relevant to tax calculations.
The following screenshots are an example of this supplemental information:
Unless you are willing to go thru the calculation to break out JUST the amount from your own state's bonds, you would select "Multiple States" for all of it
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And if you do decide to break it up...you only need your home state, and any US territories...all others are lumped together as "Multiple States" s
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IF you are going to break out your own state's $$ then you check the selection that displays the extra boxes
(Note, for a 1099-DIV, box 11, this is not allowed by Illinois, and CA & MN have severe restrictions on when it is allowed, NJ allows DC to be included, and UT residents can list a few other states...((but I only explain when necessary)) ). The amount actually from your own state's bonds would have to be more than ~$50 or $100 before it had much of an effect.
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Example is for a NC resident: