You can do a vinegar soak like the OP did, just keep an eye on it. The vinegar will start to eat away the iron. Depending on how rusty it is you might be able to get it done in a few hours or a day or so. The absolutely best thing for rust is an electrolysis system. If you are somewhat handy they can be done pretty easy, especially if you are just planning on doing one or two pieces. You can use a five gallon bucket and a battery charger. Just don't take a wire wheel or some type of grinder to it. I have seen tones of cast iron ruined by wire wheels.