Vintage dresses can be a difficult item to shop for. For one, the sizing system was on a totally different scale, and there wasn't a ton of consistency among various brands. So what's a girl to do?
Know your measurements. The most important ones are bust, waist, and hip. The rest are nice to know if you're shopping for something really form fitting online and can't try on, but otherwise those three should be enough. Measure with a soft tape measure, and don't measure tightly unless that's how you like your clothes to fit.
Now that you know your measurements, you can start looking at dresses online. A lot of sellers will tell you the original size of the dress, what it fits like (approximately) by today's standards, and its measurements flat. The measurements (again) are important. Double the flat measurement, and compare to your 3-dimensional measurements. If an item has an elastic waist, ask the seller to stretch and measure to get an idea of the range of fit. Never trust a seller's claim "fits like a 6!" without measurements to back it up.
Another thing you want to keep an eye out for is length. In a perfect world, we would all find dresses that fit perfectly and are all the right length, but let me tell you, if you are not particularly tall (and sometimes even if you are), it isn't going to happen. Go for a dress that fits your measurements up top, and either learn to sew a hem or invest a few bucks in getting it professionally hemmed. It shouldn't be terribly expensive, and you can select your most flattering length. Vintage dresses, although super neat, we're not cut the same way we are used to wearing them. They were often past the knee (midi-length), which doesn't look good on most people. Considering having dresses hemmed opens up a whole new world of dress options that, with a little work, could be perfect for you. If you have a little extra cash to spend on alterations, also be open to dresses that are bigger than your size all around. You can have them taken in at a tailor, and if you can do this, you have a much greater selection of dresses with a wider variety of colors, styles, and cool details.
After you've figured out what sizes to look for, you can really start digging to figure out what you want regarding color and cut. The best way to do this is to browse for fun without any pressure. Don't wait until you need to find something, because you'll never match the picture in your head up with something real. Peruse when you're bored, bookmarking or favoriting everything you kind of like. Then, when you do get ready to purchase, you have everything already picked through, and you can just figure out your favorites.
Lastly, think about what shapes, patterns and colors flatter you. What looks good on you depends on your hair color and length, skin tone, body type, and stature. Think about the qualities that your current wardrobe items have. What is it about your favorite dresses make them your favorite? What items do you get the most compliments on? Take those basic ideas and apply them to your search for vintage dresses. However, don't be too firm in your requirements. Just because you've never worn a particular color or style of dress doesn't mean you shouldn't. If all else fails, just look for things that jump off the page at you. If you are drawn to it, consider it. Chances are if you think it's neat, other people will too,
Pro tip: you can wear just about any color (yes, even the ones your mother said washed you out) on any skin tone with the right blush. Just apply a bright pink, coral, or bronze color on the apples of your cheeks and you're good to go.