Do you need to take wood siding off a house if your going to vinyl side it?

No, but it will not help with the finished look of your new siding. Your j channel will look gaudy next to the window trim if you dont strip the old siding, but if your going to metal wrap the trim then you can pad the trim up to eat up that space, which I highly recommend. It will help make your new vinyl siding look ten times better, or go with a vinyl trim board or wood padded on the back side to bring your trim about a quarter to a half an inch beyond the j channel face which is what it would look like new.