The main thing when you're going for a retro aesthetic is to decorate with a color scheme in mind. Pick two to three colors you like and work with those, and then incorporate neutrals (beige, white, etc) to keep things from getting too crazy. There are lots of vintage color schemes (pink, turquoise, avocado, mustard, orange, mint, and more!) that you can go with. Instead of buying vintage stuff willy nilly, make sure to pick out vintage items that have a simliar color story.
If you already have a few favorite pieces, pick out the main colors from those and work with that color scheme. If you haven't started decorating yet, start brainstorming what color scheme you'd like your room to have. Start a pinterest board to get all of your inspiration together!
If you're afraid of having things look too clashy and crazy (vintage stuff has lots of prints), maybe go with one statement printed piece and then have everything else be solids. Have a crazy vintage couch with a bold floral pattern, but then keep the rest of your furniture simple. Don't feel like you have to have everything be truly vintage. It's pretty easy to find modern pieces that can fit seamlessly into a vintage inspired room. And remember that thrift stores are your friend. You may have to go thrifting a lot in order to score great pieces, but there are often some amazing gems to be found at great prices!
The retro aesthetic is naturally pretty busy and visually dense, but that doesn't mean your room has to look like 1973 threw up in there (unless you want it to!). Vintage style is pretty popular these days so there's lots of inspiration online. The ladies at Oh So Lovely have a great decor category where they share their home decor and other inspiring rooms with vintage decor (and some tips for vintage decor). You can also find some good retro-modern home inspiration at Smile and Wave (Rachel also made a great home decor e-course if you want to sign up!), Thompson Family (and Danielle's pinterest), A Beautiful Mess and sometimes at Design*Sponge.
top two photos via Thompson Family, third and last photo via Smile and Wave.