I'm not real sure if PT wood inside of the vinyl would have been better, but I've had some concrete (and 2 half inch rebar sticks) filled posts bend during a severe storm. My posts are 10' tall (7' out of the ground) and 5" in size. I used 5000 psi concrete and the rebar as suggested by the vinyl fence manufacturer. I filled every post to within 1" of the top with concrete and rebar even though they said for "non corner" or "non gate" posts that I only had to come 3' out of the ground with the concrete fill. I still wonder if using PT wood in them would have been better, or if the wood would have snapped?
Each post took 5 eighty pound bags of concrete mix and two 10' half inch rebar sticks. I used two bags of concrete on the outside of each post, and it took 3 bags to fill each post. Other than using 4X4 PT wood inside the vinyl posts, I don't know how I could have reasonably made them stronger. /forums/images/graemlins/confused.gif I guess that a 7' tall privacy fence just catches a lot of wind. I used a come-a-long and a chain with my tractor and was able to bend the posts back upright after the storm last year. So far, they haven't moved again. Attached is a picture I took just after putting up the fence, before I did any landscaping.