Like the majority of wokbenches, mine is fine for planing but too low for cutting dovetails without prolonged stooping. My answer is to clamp a heavy backing board, as shown in the sketch, and clamp the workpiece to it. I usually tilt the work so half the dovetail-cutting lines are vertical and then, after sawing, tilt it the other way for the second set. A refinement is to draw layout lines on the backing board for reference.
I really like the main point of this tip because it's an utterly simple solution to a very real problem. Stooping to cut dovetails – or anything, for that matter – is uncomfortable. When you're trying to do accurate work, you need to be comfortable, and you need to be able to see what you're working on; in the case of dovetails, that's often a very fine line made by a marking knife. This tip solves both of those problems.
Here's what I'd do differently, however. I'd clamp the backer board in my face vise, or use long clamps across the bench, one on each edge of the board. Clamping it in the tail vise, as shown here, doesn't counter the movement of your saw. I'd also use two serious clamps on the workpiece. As for the board-tilting business, I don't do it. It just adds extra steps. Your chances of cutting perfectly plumb are no greater than cutting at an angle, so why not just learn how to cut to a line.