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You have 2 options. You can buy similar prefab fiberglass columns or you can wrap a pressure treated 4 x 4 or 6 x 6 with PVC trim boards (PVC boards are not structural) and finish it with PVC moldings. Either way your ballpark cost is ~$500 per column installed.
You can't "encase them in vinyl" except that the second option above is essentially that. PVC is a form of vinyl but the boards are too thick to just cover what you have and there are structural issues as well given that you see deterioration in the exisiting wood