First, it may not be the most tax beneficial move to realize the least in capital gains. Whether it is or is not depends on the rest of your tax picture this year (and possibly future years).
Second, Vanguard has different settings for which shares are sold in situations where you have bought shares of the same mutual fund at different times and prices. Personally I use specific ID, but there are others.
Third, if you want to minimize capital gains, the solution is generally straightforward: Sell the shares that you purchased more than a year ago (in order to get long term capital gains instead of short term capital gains) that have the highest cost basis. Vanguard will show this information to you when you go through the selling process on their website. If you happen to have shares you bought less than a year ago that have a capital loss, those would be even better to sell by your requested metric. However, remember that capital losses can only offset capital gains and up to $3,000 per year of ordinary income, so if you had a large capital loss it might end up being carried forward to future years, which may or may not be what you want.