If you ask nicely via email and give a good reason (like you have been a loyal sprint customer and will resign your 2 year contract) sprint will give you free vision. If you are nice. Yelling and *****ing about it doesn't get on their good side though...
I recomend reading up on sprintusers forums, specifically the thread titled Apparently now is the time to ask for discounts and free stuff.
You can use normal vision as a modem without any difficulty, to sprint it looks just the same as your phone usage (unless you download stuff from bittorrent or something that a phone obviousally cant and shouldnt do). I hear some of the newer phones report if you are on vision or not, if that is the case youll get a text message saying you need a modem plan.
I pay $35 a month for my entire bill, and have a broadband enabled pda that I can tether to computers without issue