I use vinegar.
I also use a lot of soft oils. The use of soft oils for me is based in cost effectiveness.
While vinegar will not kill the slime in a Castille soap it does help harden a Bastille type soap so it lasts longer and unmolds easier.
I like a low cleansing value and lots of lather. I tend to not put much value on the "conditioning" value listed in a calculator.
I use prepared vinegar water as half of the liquid in most of the recipes I make.
Obviously I do not need vinegar in soleseife ( salt brine) or salt bars so I usually remember to not waste it there.
I also use sodium citrate which by itself tends to soften soap. DeeAnna can explain the chemistry involved better than I can but I do realize that citric increases the soap's affinity for water ( makes it dissolve faster). I also like the effect that adding sugar at around a 2% ratio to the oils in the liquid. All of this means that my "lye water" is kinda like a recipe of it's own with vinegar, sugar etc added in.
All of this gives me the soap that I am looking for. Nice lather the rinses off easily, doesn't strip the oils from my old hide and a lasting bar of soap.
There are recipes with enough hard oils that can be used that will not require any SL, salt, vinegar etc...
It is worth the time to study the fatty acid properties and profiles of oils by playing with a good
lye calculator for a few hours.