I'm not sure why you would set the part in a tilting vise. By setting the part up askew, you cannot take advantage of the common plane shared by both sides of the dovetail.
I would get the proper angled dovetail cutter and set the bottom plane of the dovetail perfectly parallel to the table. Set this up carefully and do not spring the part in clamping. Use 3 point support for clamps and indicating level. Then add the 4th clamp and shim or jack the part so that tightening the 4th clamp down does not spring the part.
After that, then you also need to indicate that the carriage surface is square to the dovetail. Otherwise, you might machine both sides of the dovetail parallel, but out of square to the machine. You'll have a most unusual lathe after that
When you've got that square, then you can start to check the angled surface of the dovetail and find out which, if either, surface is correctly cut. You might need to trim both sides.