I would never use Vinegar.
There's a spot on the Wildwood boardwalk called Curley's Fries. Pretty damned good fries. Of course, I bypass the vinegar in favor of ketchup.
Although, I know people who use Mustard.
I love dipping my fries in plain yellow mustard!
It has to be French's. None of that Grey Poupon stuff for me. (blech!)
Truthfully, I prefer my fries plain with just a light bit of salt on them. Although I do occasionally dip them in ketchup and/or mustard.