Maybe others have seen what you describe but last Fall, I re-did our porch. However, my posts aren't load-bearing so I was able to remove them and 'sleeve' the 4x4s up with vinyl-clad sleeves. Then, I re-installed them. Caps can be bought for top and bottom.
If your posts are weight-bearing, then you'll need to support the porch with a temporary support while you remove the MAIN post(s) to do the job. You can still sleeve your posts up BUT don't put any weight on the sleeve. (If you do, it will 'bend/bow' AND not hold up the porch roof.)
If you're still at a great loss, my neighbor across the street aluminum-capped his posts. He doesn't have any rails and balusters inbetween his posts so it was an easy job. The only thing I don't like about it is that they dent very easily and, thus, look lousy over time.
I used Heritage Vinyl in my job. My best to ya and hope this helps.