It's not big. Fortunately the supply of content is small as well. Unlike teen which is a huge niche that's over supplied, vintage is a small niche that's under supplied. From personal experience I would also say it retains very well. It tends to be older guys who are remembering times gone by.
www.astral-blue.com/tour is a nice earner for us on not a lot of traffic.
Take a look at some of the old stars and analyze the searches and what you find to satisfy those searching.