I've been testing the XeroVest in a car -- windows down and heat off. I can report that it presents no impediment to driving whatsoever. Putting it on was easy too -- it's loose when it goes on, and tightens up when you inflate it. It's rather like a warm hug. The seatbelt isn't an issue -- just remember not to over-inflate the vest when you first put in on, and keep the lower belt below the vest.
If you were one of the thousands stranded and unprepared on the roads this winter, you'd be grateful you had a few of these in your glovebox. Just this week, nearly 100 cars were stranded on Illinois Routes 2, 17, and 231 for as much as 9 hours. The Chicago area generally had tens of thousands with no power or heat.