vinegar is effective if you are using some sort of acid dye, i don't know if it would be of any benifit using natural dyes. one of the keys to dyeing success is to prepare your fabrics to accept the dyes. pre-wash with detergent (if you are not using PFD fabrics) then soak the fabric in a soda-ash water solution. using plant dyes takes some time...it is important to give it time to 'cure'.
we have boiled walnuts for dyes; my mom and i have done some pressed flower dyeing. the best results were when we let it set for 24-48 hours. then we rinsed in cold water...dried the fabric then heat set it and let it set for another 24 hours. then tossed them into the wash. most turned out very well. a couple (reds) that i came up with didn't do so well...red is a tough color faded quite a bit to a dusty rose, but the purples really came out good and so did yellows. good luck and have fun with it. everytime i dye new fabrics i am amazed and thrilled with what comes out. always a pleasant surprise :)