Presumably the vinegar in Slacum is to raise the acidity and speed up the oxidization, ie drying, of the oil? Any chemists around to help on that?

Drying of linseed or any other drying oil is subject to the old principle of volume proportion to surface area. The more surface area exposed the faster the drying, hence the advice to use the thinnest coats possible. I have never had problems with drying when using super thin coats spread by hand. They usuall dry overnight.