What Catfish said, plus...
That crazy vacuum ball under/on the hood and its T connector are common culprits too.
If you find that everything looks good (No loose/cracked vacuum hoses, etc...), check out the condition of the climate control vacuum switch.
It's mounted to the AC/Heat control module (on the right side of it), and if your van's dash is like my '92 G20's, then there are just 6 screws to pull the cluster trim, and then 3 hex head screws to pull the control module loose from the dash.
The vacuum switch is the one that is connected to the lever that changes the airflow from the defrost, dash vents, floor vents, etc...
It has 8 or 9 vacuum lines going to it with a round, rotating vacuum maze "switch", that is actually connected to the lever.
Mine was doing something similar to yours (blowing from the defrosters and feet vents only), and after I checked everything else, I found that the switch had come apart and was leaking.
$20 later for a new switch, it was as good as new.