vinegar rinse - is it bad for your hair

No.  Acidic rinses actually balance the pH out in your hair--it's why any good or pH blanced shampoos actually have citric acid in them.  It also closes down the cuticle.

I wouldn't do it daily because too much of anything is always a bad thing, but once a week will improve your hair's health, texture and even shine.

I have to use quite strong vinegar rinses and my hair is fine.

On the contrary vinegar can be therapeutic in a lot scalp conditions like flaky itchy scalp, seborrhea and oily roots. Plus it makes the hair shine. As long as you use it with water it will be fine.
Plain vinegar on it's own is too strong to be alone on the hair that's why you should use it with water. Let's say mix in a big bottle one part vinegar and 2 parts water. Should be fine 🙂

Acetic acid in vinegar is not keratolytic - it cannot break down the keratin protein in skin and hair. Some other acids are keratolytic, for example AHAs used in facial exfoliators and body lotions. Alkaline products like bleach are actually far more damaging to hair than acidic ones, as Intempestivity says good shampoos and conditioners are acidic (be careful with 'pH balanced', that often means neutral pH). The author of this article is a scientist
http://www.naturallycurly.com/curlreading/curl-products/curlchemist-the-scoop-on-vinegar

Thanks so much for all your replies