Enter * 34

The keypad display will change to:
Exit Delay
                     34
Location 34 is a programming location that covers both partitions of a Vista 20P, so it requires two separate entries to fill the location.

Enter the amount of time (in seconds) that you would like.
(Do it twice)

The keypad will chime three times and advance to location 35 "Entry 1 Delay"...