Quite some receipes of old (pre-iron-gall) ink seem to include vinegar. It is of pale colour though, and it may or may not be corrosive to the paper. Whats it's use?
A quick google with words 'vinegar ink recipe' brings up this:
1/2 cup black walnut hull powder (or other ink herb of choice)
4 cups filtered water.
1 cup extra strong brewed herbal infusion.
1 tsp. distilled white vinegar (a natural mordant to help the color last longer and stay)
1 tsp. ... binder to thicken and keep the ink on the paper (optional – see below)
The wiki entry for mordant:
A mordant or dye fixative is a substance used to set (i.e. bind) dyes on fabrics by forming a coordination complex with the dye, which then attaches to the fabric (or tissue). It may be used for dyeing fabrics or for intensifying stains in cell or tissue preparations.
An old article has this to say concerning the use of mordants with inks(emphasis mine):
The ink first used probably was some natural animal pigment such as the black fluid obtained from various species of cuttle fish but the limited supply of this material soon led to the use of a mechanical mixture of water gum and lamp black and the characters were painted rather than written by means of a broad pointed reed As ink of this simple nature was easily removed from the surface of the parchment by the mere application of moisture it was early found necessary to contrive some means of forming a more durable ink and for this purpose the expedient was adopted of treating the mixture with some substance such as vinegar of the nature of a mordant which would penetrate the parchment written upon and form an ink not liable to fade A chemical dye consisting of an infusion of galls with sulphate of iron was afterwards used as from its vitreous nature it bit into the medium employed...
Chamber's Journal of Popular Literature, Science and Arts W & R Chambers, 1891 - London (England)