Just keep the solution at pH 2-4 for the acid phase, then pH 12-13 for the basic phase. Not sure what are the "best" values - I keep pH 3-4 and 12.5, respectively.
There are more variables, like the starting material, amount of water needed etc. So you can do it by adding small amounts of acid or base until desired pH is reached. MHRB, for example, was already acidic by itself (measured pH 4.5) so very little vinegar was needed.
You don't have to bother with vinegar percentage so much. Adjust pH for the first soak/cook - it converts alkaloids to acetates (in case of acetic acid used). The next soaks/cooks can be done with just plain water (no vinegar).
I haven't seen any difference in yield for different acidities and different acids (tried citric, acetic, phosphoric, hydrochloric).
Acetic is particularly nice as it is monoprotic and thus pH won't jump when changing the solution to basic.