For a stopbath one does not need to use a strong solution. Vinegar 1:4 in tap water is pretty mild 1:8 should work well also...certainly, I believe it would be more effective than water. You could also try a 2% solution of sodium bisulphite.

I would guess that the shorter the development time the more precision is aided by a stop bath. For instance to get predictible results with say Pan F+ in a developer at 5 minutes in a Rodinal solution compared to Delta 100 stand development of 45 minutes in Pyrocat HD solution the stopbath might be more helpful in controlling contrast variations when the time is short than when it is a great deal longer. In either case stopbath should be helpful in preventing stains etc.