Vintage has stopped being the thing you dug out of a dusty rail at the back of a charity shop and started being properly organised, labelled by decade, photographed on an actual person rather than a hanger, sized the way sizing works now rather than the way it worked in 1978. That's mostly good news. It means fewer disappointments when the parcel lands, though it also means the genuinely interesting stuff gets snapped up faster than it used to.
The thing nobody tells you is that one vintage piece in an outfit reads as interesting, two starts to read as a costume. A floral tea dress with a leather jacket looks like you got dressed on purpose. The same dress with a vintage cardigan and vintage shoes starts to look like you're off to a themed night out, even when you're just going to the shops. I keep one modern anchor in every outfit, usually trainers or a plain coat, and it stops the whole thing tipping into fancy dress.
Sizing is where most of the money gets wasted, because a vintage size 12 bears no relation to a current one, it's often closer to a 10 or even an 8, and the only number worth trusting is the actual measurement in centimetres, not the label. I measure myself properly, not the hopeful version with my stomach pulled in, and compare it against the garment laid flat. Shoulders are the real problem area, since older cuts tend to sit narrower and you can't let a shoulder seam out the way you can a waist. If the shoulders don't fit, nothing else about the dress matters.
Before you buy, check the stitching under the arms and along any zip, because that's where decades of wear show up first, and check the lining separately from the outer fabric since one can be in far better condition than the other. A small snag description in the listing is honest, a photo taken from four feet away in bad light usually means something's being hidden. On colour, I've had the best luck with navy, bottle green and soft browns, the shades that look like they were always meant to look a bit worn in, rather than bright prints which show their age in the fading and can end up looking tired rather than characterful. What you can't get around is that fit adjustments are limited and fabric that's genuinely old will eventually need replacing rather than mending, and no amount of careful washing changes that.