I've never used stop bath in C-41 development. Reason: somewhere along the line, in the dark mists of time, using Tetenal/ Photocolor/ JOBO-Russell or ?, there was a strong advisement with explanation against its use. What effect of the use stop bath would be -- I don't know .. It been a long time - and I've never tried it. I seem to remember it had something to do with the avoidance of "pinholes" in the developed emulsion.
On using diluted vinegar - Most vinegar is labelled "5% Acidity". I think that indicates a solution containing 5% acetic acid. Diluting that 1:4 with water will result in the "standard" stop bath concentration of 1% acetic acid.
One mystery I'll never understand is why so many people are so negatively affected by the smell of what would be an extremely weak salad dressing..??