Congratulations on your upcoming wedding. I applaud your decision to purchase a new outfit for the big day. After all, your fiancÃ©e will likely end up spending even more money than you will on her outfit, and she will probably only wear it just that once (although my wife and I had a spectacular 1st wedding anniversary where we dressed up again in the same outfits and enjoyed dinner at one of the best restaurants in California. In fact they actually lost our reservation but seated us anyway because who can turn away a woman in a wedding gown?).
Both midnight (or just plain dark) blue and charcoal gray would be fine choices for your suit. I agree with Manton that you should go for a double breasted suit if you choose midnight blue because a single breasted suit (especially a peak lapel SB with a vest) would look too much like a wanna-be tuxedo. But my preference would be for a SB gray suit with the now-exotic DB waistcoat that Manton also suggested. However, in light of the festive nature of the day I prefer Sator's suggestion of a non-matching waistcoat. Buff is a lovely creamy yellow color (avoid any fluorescent Post-It Note yellows, though) and would look great on most complexions. Traditionalists may scoff but I would actually go one step further and consider choosing a silk waistcoat that compliments your fiancÃ©e's choice of bouquet flowers. Just last month I attended a High Church English wedding and the attendees' morning suit waistcoats were a rainbow of rich scarlets, purples, and golds as well as the more typical dove gray and buff. As long as you choose an attractive rich color in dupioni silk you needn't worry about being embarrassed to show your wedding photos 20 years down the road. You could make a gift of similar SB vests for your ushers to wear with their best-fitting dark grey suits so that you match without forcing your best friends to all wear ugly ill-fitting rental suits. Don't stress too much if their dark grays are slightly different than yours - a row of men in attractive well-fitting suits is preferable to a row of men in identical ill-fitting suits.
You could actually wear a waistcoat with the DB navy suit, too - plenty of men did just that in the 20's and 30's - but that is a lot of material to have strapped around your waist all at once. So the charcoal gray would be my preference. Both of my 3-piece suits are 3-button cut just low enough to show 1 or 2 buttons of a SB waistcoat and still leave room to show off a necktie properly. But for this event you will want to properly display your beautiful silk DB waistcoat so a 2 button jacket that buttons at your natural waist would be best. Some people recommend no vents for formal suits (they supposedly have the cleanest line) but they wrinkle when you sit and look atrocious when you reach into your trouser pocket. Single vents don't look as good as double vents from behind (as in, when you are standing at the altar) and again look ugly when you reach for your car keys. As for lapels, notch lapels are fine but I think peak lapels look better and in a dark charcoal gray fabric most people won't even notice them - preserving the post-wedding flexibility of the suit. Make sure the peaks don't point straight up or end on top of your shoulders.
If your tailor doesn't offer appropriate waistcoat material choices then ask him to give you your torso measurements and look online for a waistcoat merchant whose address ends in ".co.uk". The Brits will have lots and lots of made-to-measure choices.
Here's one final idea: have you considered a stroller? It adds a nice formal touch yet you could probably get away with wearing it more frequently down the line than you could a full morning suit. Especially if you treat it as an odd jacket and pair of patterned odd trousers. Few Americans would know the difference.