I have good success with the Mr. Clean Magic Eraser on some tough stains. Softscrub and a tooth brush can work wonders. Make sure to reapply a conditioner and go slow.
Was it a fountain pen? Just kidding. I was going to suggest softscrub also. You can also try WD40 too. Make sure you don't use harsh cleaners and flush with clean water to rinse.
Isopropyl Alcohol works without hurting the vinyl. It takes a couple of days and a couple of applications to completely disappear (from my experience).
I'll second the Mr. Clean Magic Erasers. I've cleaned off all the vinyl in my cabin, despite a heavy coverage of nicotine tar. Scrub until the pad starts to disintegrate, then wipe with a damp cloth. Condition after it dries.
2005 300DA, Garmin 740s w/Vision Charts and GMR18HD RADAR
Engines
5.0MPI w/BIIs, Kohler 5E Generator.
Another good one is butter. One thing to note though... if this is on an outside seat that is/can be exposed to sun... Spare your vinyl, and don't go after the ink too hard. The sun will fade the ink pretty quickly. A tech working on my boat got blue ink on my white helm seat, and I was seriously worked up about it. After trying a few different cleaners and tricks from on here, nothing was getting rid of it. Came back the following weekend and it was gone... entirely gone. (Boat is at a private residence so I know no one from the repair shop came and tried to work any magic on the vinyl).
If you are like me, it will eat at you when you see ink there, but give it a week or two before going after the ink with chemicals/abrasives, your vinyl will thank you.