The most common place to find a mitred vent will be on Suit jackets for men, although some ladies jackets will have this feature. The jacket may have imitation buttonholes and buttons. It is important that the buttonholes are imitation and not normal buttonholes that are cut. If they are cut you will need to alter the jacket from the sleeve cap. This will be covered in How to Shorten Sleeves on a Lined Jacket from the Sleeve Cap.
Whilst this is a complicated clothing alteration, you do not have to be a professional to do this alteration.
The eBook includes illustrations and step by step instructions to complete a successful alteration.[eB104]