What can I do if a buyer stops making payments partway through a vendor take-back note?

What's available depends heavily on how the vendor take-back was actually documented and secured at the time of the sale. At minimum, you generally have an ordinary contract remedy on the note itself — the ability to accelerate the outstanding balance and sue for the amount owing once the buyer is in default. If the VTB was secured, commonly through a PPSA registration against the purchased business assets, or a mortgage if real property was involved, you generally also have remedies available to a secured creditor, potentially including realizing on that collateral, subject to whatever process the security document and the PPSA require.

An unsecured VTB — just a promise to pay with no registered security behind it — leaves you in a considerably weaker position, closer to any other unsecured creditor chasing a debtor. This is exactly why proper security matters so much when a seller agrees to finance part of the purchase price rather than taking full payment in cash at closing; review your specific security documents to see what's actually available to you now.