I installed a vinyl fence last year. I got an great deal of the vinyl fence and posts at the local Home Depot. The vinyl fence posts were too short - hey the were $3.00 with he caps!
The result was that I ripped down 4x4's on the table saw and extended the post by about 16 inches.
I did not install the fence with concrete for most of the posts, I did anchor the corners in 160 lbs of concrete.
On the windward side, the fence moved. Those posts have a lot of gravel below and around the posts. I wanted to add concrete and considered re-plumbing the posts with respect to corner posts and adding concrete.
These posts extend about 28 inches below grade. I really don't want to re-excavate the entire hole. I was thinking of excvating the top 16 inches and adding about 120 lbs of quickset per post.
I'm not thrilled with using concrete but that appears to be the only solution. Looking for suggestions as to how to set concrete for these posts.