Chevy's are bad about AC blowing to floorboard?

You have the right idea about it being a vacuum problem as most of the time it is with this symptom. There is a 1/4 vacuum line that connects a source of engine vacuum to the inside of the car to control which blend door and vents open or close. Most of the time this line will rot out and won't deliver the vacuum to the inside of the car. Find the line and replace it. Most of the time, the line is run through the penger side of the firewall. The vans mostly had lines run straight off the intake manifold, while late model full size trucks actually had separate small canisters that were mounted near the heater core tees on the penger side inner fender.