https://en.wiktionary.org/wiki/vender

https://www.dictionary.com/browse/vender

https://www.vocabulary.com/dictionary/vender

https://www.vocabulary.com/dictionary/vendor

You can find it in Francis Bacon's The Elements of the Common Lawes of England (printed 1630) - https://scholarship.law.wm.edu/oldelawebookes/28/

That said, vendor has become more and more the standard spelling in legal texts.