Vest and cummerbunds are usually not required for prom, but this will also depend on the type of suit you've picked. There are some styles that just look better with a silk vest beneath them or a cummerbund resting on the waistband. If you're unsure, simply ask at the store you're shopping at. However, a simple suit jacket with a great shirt and tie is usually all you'll need to look perfect for prom night. 

Cost can be a big factor when thinking about renting vs. purchasing. To rent a suit costs on average around $150. You may want to have it dry-cleaned before and after prom to make sure it looks crease-free and is returned in top condition. Keep in mind that you may be held liable if you accidentally damage it on the night too! Buying a suit can be a good option to consider if you think you may wear it again for further events, such as weddings and interviews coming up. There are many great stores with affordable suits in stylish skinny or fitted looks that suit an array of occasions. You can always style it with different shirts and ties to make the outfit different for various events. A suit that fits you well and is made from high-quality materials should last for many years and can be an asset in your wardrobe. 

Choosing the color of your suit is very important. The most popular choices include black, grey/silver, tan/brown, navy, and lighter blue. The color of your suit shouldn't clash too harshly with your date's dress. For example, a navy suit and royal blue dress might look a little uncoordinated. If you don't know what colors match, the best choice is always black or grey. These neutral tones are easier to match up with most dress colors, and you can use your accessories to coordinate with the dress instead.

You should pick a pair of formal shoes, made from leather in black, brown or grey to match your suit color. Styles such as loafers and simple dress shoes are all that is required. These can be worn again for future interviews, college applications, graduation, etc. Make sure you pick a neutral shoe that works well with semi-formal pants. 

A simple skinny silk or matte finish tie in a block color is all you need for prom. The color should contrast a little against your suit, or it can match if you pick a lighter shirt. Stay away from patterns or anything too novelty, a tie should just add a little color but not be the main focus of your look. You could also opt for a bow-tie in a color that matches your date's dress or accessories. 

Your shirt should be a long sleeve formal dress shirt, with a stiff collar and a full button-up front. Most guys pick a simple white shirt as it's the easiest to coordinate with any color suit. However, depending on your suit color, you could also opt for silver, deep green, burgundy, black, etc. Make sure that if you do pick a colored shirt, it doesn't clash with your date and matches up well.

There are other prom accessories which are completely optional but add a stylish twist to your outfit. You can think about adding a pocket square for example (a colored piece of fabric which is folded and displayed in your top suit pocket). You may also want to make sure your socks match the outfit in case they are shown as you walk. A great watch and a leather wallet are always great additions to a formal look.