Ok I'm late 60s and have also worn vintage most of my life although it sounds like we wear different things. I'm 5'3 and generally size 8-10 but not particularly interested in dressing for my body shape (tried it and didn't get on with it).
I mostly wear a mix of vintage and modern, e.g.
1940s/50s/60s jackets. Yes they're getting hard to find but I'm still wearing some I've had 30+ years - they're built to last! I have very narrow shoulders so they fit better than contemporary ones, even when the design is similar. I usually team with wide leg jeans or ankle length culottes.
The same jeans/culottes with a loose fitting sweater-vest (often mens') over either a T shirt or cotton shirt. Or if I'm scrubbing up, with a silk shirt.
Sometimes I swap the jeans or culottes for either a full 'new look' style skirt (but modern) or a straight-ish midi skirt (modern) or a pleated wool 'Barbara Woodhouse' skirt (vintage)
I rarely wear dresses as they don't fit properly top and bottom. Same with jumpsuits - wear the Sainsbury's one, I'm very jealous!
Like you, I mostly get my vintage things from fairs, plus we have a couple of charity shops with vintage rails. (I'm on very good terms with them!) More recently I've bought a couple of jackets from Ebay.
A lot of my modern clothes are second hand. I particularly like Margaret Howell and some Toast (I keep an eye on their Reworn site which is more reasonably priced than Vinted plus you can return). Also John Smedley for knitwear. But otherwise I'm not loyal to any label
At the moment, if I'm buying new, it tends to be Uniqlo - I like some of their collabs, especially the C range. Also buy bits from M&S and I still have a soft spot for H&M, even though the quality's gone to the dogs.
I'm not sure how helpful this will be as we like different things but I didn't like to ignore a fellow vintage fan!