With no jacket, a vest with belt over the bottom would look fine. But as stated above, the buckle makes the bottom of the vest stick out--I tried it, though advised not to! I'm a little hard-headed--and it just isn't needed. Without a vest, a buckle sets off the ensemble. Though Captain Farquharson wears both, and pulls it off well, it's rare to see.

I think your level of formal dress should match the level of the general gathering, so if you aren't sure, just ask. (My own personal policy is that it is better to be overdressed than to be underdressed. You can always shed a jacket or tie, but it's hard to grow one!)

For black tie, a black argul jacket with either a black bow or a long, plain black tie is fine. In fact that's about as "formal" as anyone in the U.S. gets anymore. If it's just jackets and ties during the day, make yours match and stick with brown leather if you have such. For evenings, black leather is always proper.

Strictly speaking, a tuxedo is "black tie," that is, not formal and restricted to after 5:00. So for a day wedding, you'll probably be fine in any combination of a jacket and tie--or less if everyone else is in shirt sleeves and sneakers.

My own personal thinking is that Scottish attire should not become the subject of farce, nor should it detract from the proceedings. So, I never wear my doublet and lace to a wedding; it would not be cool to look better than the bride!