I’m guessing this is your last major operation on the part?
I’d personally put this part on a tooling plate, blocked in with clamps on the base, and held down from the top at the dovetail with some round stock (or a made to fit piece in the dovetail)
View attachment 10416
Another option (if you were to start over), is to use larger stock, leave the excess behind the dovetail to clamp into the vice, then mill away the excess and then cut the dovetail.
One thing I like and hate about machining is the 'how would I do this ....' usually it's always too late in my case, but each time I learn a little more about how I should have done it .... and hopefully my recall is good when I need to do a similar task next time.
Brad