I don’t have a picture of it (I will try to remember to take one next time I use it), but I have a dovetailed 90 degree profile that I use for things like that. In the situation you show, I would clamp the profile to the larger workpiece, and then capture both in the vise (or against the front of my workbench with a clamp). The tail part is then positioned and clamped on the other leg of the profile to mark the dovetails. Because the exact 90 degree is set by the profile, rather than the vise / work bench top, it does not matter if the clamped SA in the vise is not exactly vertical, because I have the ninety degree from the profile.
Tip: When you make profiles for things like this, always make them in pairs from a single well prepared board, so you can use the second as a spacer to avoid racking / deformation of the work piece.
Colin, Czech Rep.