Conor,
My experience with extremely rusty/stuck bolts or fittings is that it often takes a multitude of different strategies to break it loose. Often one thing or another is simply not enough. Soaking it for a while in vinegar is a good thing to try, but if it doesn't free it after a couple days, continue trying other tricks and strategies.
Usually, heat is your friend, as it causes the metal to expand and then contract again while cooling off (SLOWLY!) This action can break free rust deposits, etc, sometimes. Then when it is cool, hit it again with PB Blaster, Kroil, or similar. Then maybe try an air impact with a blunt tool, against the center of the end of the stuck component, to try and shatter the rust crystals. And then back to the vinegar bath, or the PB Blaster, or etc. Then, maybe back to the heat.
IF you don't have serious heat available to you (IE: acetylene cutting outfit, etc), then you may be forced to take it to a machine shop or someplace which does have heat available.
The most important factor in loosening fittings that are SERIOUSLY stuck is patience. Don't get impatient and break something. Every time you hit it with vinegar, it will reach a LITTLE farther in, and every time you hit it with penetrating oil, it goes in a little farther. Time is your friend.