Vender has been a published word since at least 1596 according to the full Oxford English Dictionary.

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.

Google ngram viewer has vendor at two orders of magnitude more usage. Personally, I don't think I've ever seen "vender" before.