I have to dispute the J.Herbin neutral claim, since I just took my pH probe, calibrated it with distilled DI water, and tested six of my J.Herbin regular lineup inks and they all read below pH 4, so definitely acidic and not neutral. Most of Waterman, Aurora, Montblanc and Sheaffer lineup of inks I have also read in that same ballpark of weak acidic pH.
For vintage pens, J.Herbin and Pelikan are safe and dry inks, so less chance of feathering on cheap paper. Other than that, Montblanc, Waterman, Aurora, Parker and Sheaffer modern inks are all considered safe for vintage pens with a few singular exceptions in their lineups.
Avoid Japanese inks inside vintage pens, especially ones that come in contact with celluloid directly inside the pen (Vacumatic), as they are very alkali/basic in nature, reading as high as pH 9-10 for the Iroshizuku lineup.
Parker 51 I believe is the sole exception of a vintage pen that can use just about any ink, and was originally designed to take the alkali superchrome ink that even had almost as high as 30% alcohol in it too. However, for the sake of preservation, avoid using Japanese inks in those Parker 51s too.
WRONG, wrong, wrong! Your data on the Herbin and other inks is useless. Calibrating your pH meter with distilled DI water is not correct. You need to use a buffer (or two). The pH of the water you used can be easily affected by things such as carbon dioxide (which redily dissolves in stored water) and other factors. You need a buffer present to maintain the pH in the face of these various factors.