Having done a tie post as part of my series on knitting for weddings, the next logical step seemed to be to do a post on waistcoats for the groom and/or groomsmen. I was looking forward to this post as I expected to find a number of nice waistcoat or vest patterns. Alas, they don't seem to exist. There are quite a lot of waistcoat patterns available, a number of which are nice enough to wear to the office with a shirt, tie, and trousers, but not really any that looked stylish enough for a groom.
The one above is the only readily available pattern I could find that looked like it might do. The design is called "The Waistcoat He Wants", it was designed by Feature Knitting Designs, and it's a free pattern.
If that waistcoat pattern doesn't do it for you, there are two routes you might take. One way is to track down a vintage waistcoat or vest knitting pattern that you like, and use that. Old style vests were much smarter than the casual, relaxed fit vests you see now. I'd recommend looking for a sharp cut, some detailing such as welts, and a shaped bottom, as these things elevate the design.
The other possible way to make a waistcoat fit for a groom is to pick out a waistcoat sewing pattern that you like, make a rectangular length of knitting in a fine gauge but in any colour and pattern you like, then take it to a tailor to be made up. I'd consult with the tailor about the project before doing any knitting or even buying a pattern to make sure s/he is able and willing to do it and to get advice, unless you are a very good sewer, in which case you might tackle sewing the waistcoat yourself. When I was researching patterns for the first post in my series on twentieth century knitting patterns and I hoped to find a great waistcoat pattern, this is what all the Edwardian waistcoat patterns I found said to do.
And I'll just throw in about the only item of wearing apparel I found on Ravelry that was specifically designated as groom wear, Groom's Socks to Prevent Cold Feet, by Eline Oftedal. It is kind of a fun idea for a gag gift, but I can't imagine any man of my acquaintance actually being willing to wear these, and it seems like a lot of work to do just for the sake of a joke. However, if you know a prospective groom who could use such socks, the pattern is available as a $5.99(USD) download.