Manufacturer pairs 6 ft tall vinyl fence with 8 ft posts - should I find 9 ft posts?
I'm about to put some of these in myself. I believe the manufacturer's instructions answer this question in Step 2c, shown below. Note that the post does not extend the entire depth of the hole, but the hole is back-filled with concrete to a 30-inch depth before placing the post. That way, the concrete holding the post extends past the frost line, even though the post itself does not.