This is a great question. A good vise will indeed support long boards within the jaws of the vise and the edge sitting in a planable position above the jaws. If the board is too wide and makes the board too high for comfortable or accurate planing, clamp the end in the vise and then clamp the board to the benchtop at the opposite end with a sash clamp. This is very simple to do. The only additional thing you will need is a packer the thickness of the vise jaw and liner, so that the board is parallel to the jaw. Alternatively you can add holes to the other leg face and put support dowels there. I often clamp a post to the board if the stock is too heavy for the vise alone. For boards and sticks, I use an alternative method using clamps in the vise. You can see the link here.