As far as I understand it...spirit dyes are aniline based...meaning fine powdered pigments are suspended in spirits...some of the color permeates the spirits and sinks into the leather, but after drying, some residue remains on the dyed surface as a powder...this causes bleed from aniline dyes...sealers can be used to help reduce the problem...or wiping down with a solvent to remove the powder residue...however, both spirits and solvents are bad for leather and can cause drying...especially on cheaper grades of vegetable tans which are dry to begin with...
Water based dyes are similar, only they can be rinsed with plain water...but they will bleed if not sealed, especially when wet.
Vinegaroon type dyes actually chemically react with the tannins found in vegetable tanned leather...this is a chemical color change and should be color fast, though prone to fading...and the ph of vinegar used to make vinegaroon dye can also adversely affect some cheaper or thinner grades of vegetable tanned leather...the ph can be balanced with soft water rinses, but because leather is organic and each hide reacts differently, no matter what dye you use, results will tend to vary a bit...
I really think the old oak tanned leathers emerged from the vat a deep reddish brown, and that they were used that way...surface color could be blacked with grease and soot or vinagaroon dips...but thats about it...today we are using much lighter colored vegetable tanned goods, and are forced to color them to replicate the looks we want, and believe me, its all one big learning curve once you start that...
commercially dyed chrome tans are all over the map too...those leathers emerge from the chromium bath a sick blue...then its all vat dyed and drum softened from there..and depending on the tannery, chrome tans may also bleed, shrink, or even rust steel that is in contact with it...which is really bad for holsters and knife sheaths...so yes, buyer beware for sure!
I doubt this helps much, but at least you will have an idea about whats happening to your leathers maybe...as for sealing, I use plain ole boot wax...
tc