if you are using purchased fabric printer sheets you should use the instructions on the sheets you are using for the best results. they are not all created equal and some need to be treated differently than others.
check out the Dharma Trading company web site; they offer lots of instuction and help for printing/dyeing your own fabrics. vinegar is usually only used if using an acid dye. for non-acidic applications the vinegar has no effect and you are just wasting your money.