<<Disclaimer: Verify this information before applying it to your situation.>>

About a dozen people wrote back saying they had never heard of using
vinegar to wash off lettuce, including a few who worked/had worked in
several different restaurants. I did get one response who claimed that
it was a common practice to kill bugs when first brought in off field.

Several people mentioned that they thought white/distilled vinegar was
on the safe-list, and several more say they have no reaction to vinegar.
Apparently this is a real issue of debate whether or not vinegar has
trace amounts of gluten.

I definitly have reactions to vinegar (except for balsamic, red wine, or
cider vinegar) but I am also very sensitive to gluten. On the other
hand, I have other allergies so it's hard to tell sometimes whether a
bad reaction is due to hidden sources of gluten or another allergen.

Anyway, in summary, it doesn't sound like a real common practice but
those of you who are very sensitive to vinegar may want to ask your wait
server each time if they use vinegar on their lettuce...