In my humble opinion, there isn't a hard/fast rule about anything (well not much of anything). There are those that say you should never wear a belt with a vest. I think the logic is that the vest hides a bit of the belt and it looks too cluttered.
I think it makes sense to see how it really looks on. There are different vests and different belts. If you like the look, then fine, if not then change it. I go with what I think looks right for the particular occasion and don't have any hard/fast rules, except never ever ever wear a plaid shirt with a tartan kilt!

A long tie with black jacket to me would be a level of formality just like a black suit and black tie. Very dressy, suitable for most anything except the most formal of occasions. I would say that if you are just attending the wedding and not the groom, it would be most appropriate.