I called a bus company to find out about bus routes (info in a previous thread I started) and was a little surprised to hear the man on the other end use 'vos' when speaking to me from the very beginning of the conversation. I would normally expect the person answering phone calls for a company to use 'Usted'. So they must be a friendly bunch
A good rule of thumb for using either tu or vos as opposed to usted is to let the native speaker set the tone and then you just follow his or her lead.