Can't help you there I'm afraid, but I can now tell you that adding the salt increases the ionic strength of the vinegar, apparently and according to Google, so with all usual caveats.
Results 11 to 13 of 13
18-04-12, 23:01 #11
18-04-12, 23:15 #12
18-04-12, 23:27 #13
I found two results on the first page of my search mentioning increases in the ionic strength and decided that was as much as I'd ever need to know about the subject.
I can testify that it's a good method for cleaning up a wire in a pinch, and that's good enough for me.
I didn't do a salt and vinegar versus vinegar only comparison as I only had one wire to clean, but will try vinegar alone out of curiosity the next time I need to clean and solder something that's not really worth cleaning and soldering. If it's less effective than the salt/vinegar mix, I shall be able to say, 'We'll add salt! It will increase the ionic strength of the vinegar, don'tcha know.', even if that is actually utter tosh.