Vinyl Fence on Deck - How to stabilize or strength the Post Support
The most appealing option would be to remove the 4x4s and buy ones that are a bit longer. Cut a 3 1/2"x3 1/2" hole in the decking and create an assembly similar to one in the image below. 

You could also use the same posts but shorten the height of the fence by cutting the white material and dropping the 4x4s below the deck as previously mentioned. 


http://embed.widencdn.net/pdf/plus/ssttoolbox/zqzekua0ld/F-DECKCODE20-p029-032.pdf