How do I delete an old invoice from a vendor when the books have been closed for that year?

Why does it need to be deleted? (bad debt, incorrect amount, etc... )

When your books are closed at the end of the year you are only closing Income Statement accounts, otherwise known as a temporary accounts. I.E. Revenues and Expense accounts.

Accounts Payables is a Balance Sheet account otherwise known as a permanent account and will not be closed.