First things first, toss the RIT in the garbage. The money you paid for it is not worth the grief you will get from it. It is an all-purpose dye that dyes all-purposefully terrible(well, maybe except for nylon and FOE so I've heard).
You are seeing soda ash(sodium carbonate) recommended in relation to use with Procion MX dyes wh/ are fiber reactive dyes. These are a class of dyes that dye natural fibers, cotton in particular, especially well. They work by bonding on a molecular level with the celulose molecules in cotton(proteins in silk and wool). So they actually become part of the fiber itself. Soda ash acts as the catalyst that allows this reaction to occur. Without it, the dye won't bond with the fiber.
You can buy soda ash online or find it where pool supplies are sold. Ph Up is 100% sodium carbonate
Vinegar or other acid is used with protien fibers(wool and silk) and nylon when using a class of dyes called acid dyes.
Salt does nothing to "fix" dyes to any fiber. Though it does aide the dye process when you have a high water to dye ratio i.e. vat/bucket dyeing.
And I am not joking about tossing the RIT in the trash. Nothing will fix it or keep it from washing out little by little. Its not worth the hassle.
Follow the link in my siggie to the Tie-dye forum. There's lotsd of info about tie-dyeing and dyeing in general. There is also a tie-dye wikipedia(an online encyclopedia of dyeing) linked to the forum.