You cannot make a late contribution ... don't know how Vanguard could have allowed it at all ... check to see if they made it a 2019 contribution.
Per the IRS :
When must I deposit the contributions into the SEP-IRAs?
You must deposit contributions for a year by the due date (including extensions) for filing your federal income tax return for the year. If you obtain an extension for filing your tax return, you have until the end of that extension period to deposit the contribution, regardless of when you actually file the return.
If you did not request an extension to file your tax return and did not deposit the SEP plan contributions by the filing due date for that return, you are not allowed to deduct any SEP plan contributions on that year’s return. The contributions may be deducted on the following year’s return.
If you improperly deducted SEP plan contributions on your return, you must file an amended tax return as soon as possible.
https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-seps-contributions
