Odd vests with suits are done. They're a little atypical, and tend to work in two ways: as a touch that dresses down the whole rig (calling back to odd vests with country clothing), or as something that dresses up the whole thing (calling back to a morning suit). Consider which one yours is doing. I think I'd rather see a mid-gray vest with a navy suit.
Thanks! Sadly, I don't have a mid grey vest and the trousers on my navy suit are at the tailors till next week.
I'll try out the ensemble tonight and see how it looks. The event is black tie optional, but I think a black bow tie would look rather out of place with the fit and will either wear a patterned black bow tie or a red paisley bow tie.