We love Dapper Day. As many have already said, you don't have to buy anything special or dress in vintage clothing. The intent of Dapper Day was to encourage people to 'dress in their Sunday best' like parkgoers did in Walt's time. You are free to interpret what dressing nicely means. Many of us love to explore vintage looks but it's not the only way to go. A lot of people enjoy coordinating so that their party has a 'look', like a particular color, a theme (we once used Club 33 as inspiration), or just a feel. A few years ago, we saw a group dressed as Disney trash cans LOL. That is, their tops each had the design of a different trash in
Disneyland, which if you have ever looked, are all different designs. But how you can interpret the intent is very broad, like adding a bow tie or fun hat. And the Expo is a lot of fun, held right inside Disneyland Hotel. You can walk around and look at all the vintage things, take pictures with their photo ops, watch dancers, go to a car show, and so on. They sell really unique items there, too, often Disney-inspired.
Anyway, don't be afraid of Dapper Day. It requires very little to participate, you can go all in, or something in between. Attendees keep the event going which is good for everyone; the event expands, people have fun, and the great hotel rates are a bonus.