Which is preferred?
I've always thought that vendor was the only spelling. The question was brought up by a typo, which the Word spellchecker did not correct.
The spelling vendor is the standard spelling. The New Yorker, as part of its bizarre house style, uses the spelling vender. No one else does, besides those trying to emulate The New Yorker’s style.
Of the 45 examples in COCA, only 17 were actual uses of the spelling vender outside of The New Yorker (compared with over 2000 examples of vendor, a ratio of over 100 to 1). Two were proper names, eleven were from The New Yorker, and fifteen were in foreign languages.
These two words mean the same thing but actually have different etymologies:
"Vendor" is preferred, but there is nothing wrong with "vender"