Holding boards for hand-cut dovetails is a problem. It is one of the few times where one is sawing various angled cuts on the end of a board. Boards saw best when securely held. The usual wood vises of the cast iron sort that most of us use these days have two bed ways and an acme threaded rod to operate the jaw. This arrangement interferes with holding a piece of work vertically. If you do stand up the board in such a vise, the vibration occurring from not clamping close to the end makes sawing ineffectual. It is interesting to note that bench vises used before our modern cast iron type allowed for vertical board holding, in concert with the practice of hand-cut dovetail construction.