Hi mkdale,
The rules for calculating required minimum distributions are complicated and depend on numerous factors. If you have questions on how Vanguard calculated the RMD, I would ask them to explain their calculations.
Based on the information you provided, you appear to meet the qualifications of an eligible designated beneficiary, because you are not more than 10 years younger than the IRA owner. This qualifies you to use the lifetime distribution rules that were in effect prior to 2020. The life expectancy for a beneficiary who is age 78 is 12.6 years according to Table 1, Single Life Expectancy, so the Vanguard calculation appears accurate. (You may be using the incorrect life expectancy table, because I see a life expectancy of 22 yrs on Table III, which does not apply in your situation.)
I’ll provide the link to IRS Publication 590-B, which provides guidance for Distributions from IRA accounts.
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Kimberly, CPA for over 30 years
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