Having used 5C on mill and lathe for many years, I personally wouldn’t bother.
With a small V block and an engineers square, you can do the same thing that they do but with round, hex or square bar, just using your mill vice.
The most useful 5C item I use on the mill are the pair of collet blocks, one square, one hexagonal. Just grip them in the vice, set horizontal or vertical or even at an angle. Much more versatile.
5th item down, 5C collet block set.
John