Changing iron(III) acetate to iron(II) acetate
You need a reducing agent that will reduce iron(III) back to iron(II). Actually finely distrubuted iron could be the best reagent to do this, as it will not introduce any contaminants. The reaction taking place is 2 Fe(III) + Fe = 3 Fe(II).

Of course the iron used should not be rusty. I am not sure that you can easily get hold of fine iron powder, but I would give a try boiling the vinegary solution with fresh iron shavings or steel wool and as little contact with air as possible. The color change should be visible to greenish from the red-brown of the iron(III) acetate. 

Actually, during dissolution of Fe in acetic acid you should get iron(II) acetate predominantly until the Fe runs out, but usually scrap iron is covered by iron(III) oxides/hydroxides, and these also dissolve in the vinegar. When the iron(II) solution is in contact with oxygen in air, it oxidises rather quickly. It is a good idea to start with a batch that was not left standing for weeks.

Other reducing agents work quicker than iron (e.g. tin(II) salts or zinc powder), but these introduce other products that would interfere with your dyeing.