It means "add the source for your dependencies to your own codebase and build them with your build system".

Thanks. Not an appropriate use of the noun "vendor," so I would never have guessed that.

It's the verb vendor: https://en.wiktionary.org/wiki/vendor#Verb

"Vendor" isn't the verb. The verb is "vend."

That's a different verb, with a different meaning.

Did you read the link?