[Cary Grant]

I don't see why it couldn't be worn as is with a navy suit.

Agreed- depending on the material.

I bought two suits a few years ago in a very similar material. One is mid gray and the other a steel blue (You might calli t navy but it's a little lighter).

My tailor encourged me to try treating them as separates. It just so happens I'm wearing the blue trou with the gray coat today. But by far the best look is the gray vest with the blue suit.

I woldn't be doing it if the suit was, say, a 150's silk/wool and the vest was flannel, for example.