I'm so sorry! Eventually all rehabbers have ink horror stories to share. Ink stains are really difficult and sometimes there's not much that you can do with them - and sometimes it's best just to leave them alone. Also, there are different types of inks, and when you are working with vintage bags that may even include fountain pen ink.
At this point I think your best option is to try and dye the bag a dark color like black, navy or maybe dark brown. Did the stain soak through to the inside? If you decide to have the bag dyed by a professional, tell him that you also want the inside dyed dark.
Sone rehabbers will decorate bags to hide the stains, like attaching leather flowers or using paints to create a design on the bag. I've never tried any of those things so I don't know much about it.
Just a word of advice for the future. It's usually a good idea to try and test a product or chemical in a hidden area of the bag, like an inside leather seam. It probably won't tell you if the ink is likely to run, but at least you will see whether the product will harm or alter the appearance of the leather.
Sorry that I don't have any better suggestions but maybe somebody else will have some good ideas.