Tyvek is not a vapor barrier. That said, the felt will stop moisture and help stop air movement. It won't hurt to add tyvek and if you are in a windy area, it will probably help.
If it rains alot, moisture will get behind any kind of siding so it will also help in keeping moisture off of your T&G cedar. Its about a buck ten per running foot so go for it if you want.
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