It's not going to do much. Vinegar won't attack old ink as well as dilute ammonia. Get a bottle of cheap household ammonia and dilute it 1:10 with regular cool to lukewarm water (not hot water, hot water is bad for ebonite feeds and will turn 'em brown) Vinegar is an acid, which doesn't really respond with dyes, and acetic acid isn't very strong. You'd really be no better off than with water, and vinegar will leave a much more pervasive odor, whereas ammonia odors will disappear within moments. Ammonia is a mild base and works MUCH, MUCH better (like damn near magical) on dried ink. I was a skeptic until I tried it and ammonia just chews up old ink like magic.
Dish soap won't do much, it is a mild base as well, but it's more about the polarity than the basic-ness, and as such, won't really grab onto anything fountain pen related.
rinsing the barrel depends on how much the sac disintegrated and if the pen is translucent enough that you can see anything. I'd rinse it with water after getting the sac out and see if there's any ink coming with it. If so, just flush it well with water as long as the barrel is opaque and any potential stains or minor residue, as ammonia isn't GREAT for pens, though it's mostly harmless and won't really hurt anything, the less you do, the better.
So, long story short, don't use vinegar for several reasons. Use 1:10 diluted ammonia, it will get sh*t done. And just rinse the barrel with water unless it's a translucent barrel with some staining or visible chunks of dried ink, in which case use ammonia.
And don't use hot water, it turns ebonite brown. Lukewarm at most (100 degrees max)
Don't knock out the nib/feed unless absolutely necessary with vintage pens.
A cheap $30 ultrasonic jewelry cleaner is a godsend for cleaning old crusted out ink. Just don't use it on lacquered stuff like urushi coated pens.
Edited by Honeybadgers, 11 July 2019 - 23:52.