Having had the experience of dying fly tying material in mass, when l Owned the SLF dubbing co, l can tell you that any material that contains the residue of fat content is a problem. Be it fur or feather.
You have to remove all residue of matter from the material before you subject it to a dye bath, or you will not achieve good color definition.
As the process of heat used to dye causes the fatty residue to leech, and it is very difficult as such to boil it out.
The fatty residue will mix with the dye bath in solution.
In fact you will likely destroy the property of the material if you do subject it to intense boiling temperature for long periods of time.
Veniard dyes are ok, but there are dyes out there that are way better and contain different chemical content which works better for certain materials. Unless you are in the business and purchase in large qty you will not be able to obtain it.
Some of those dyes also contain toxins and should not be used, unless in a controlled situation.
Same also applies for some of the fixing agents needed for those dyes.
In the case of Veniard dyes, which by the way set the standard years ago so far as fly tying material color, you can use both acetic acid and salt.
Most vinegars contain about 5% in solution of acetic acid.
Many of the chemicals that you need to prep material, before subjecting to the dye bath are not easy to obtain, one of those being formaldahyde, in high percentage solution.
Deer hair is