My first waistcoat came from a fancy dress rail, polyester with a shine like it had been laminated, bought for a wedding I was a guest at rather than a bridesmaid in. It taught me that a waistcoat is doing one job, to sit flat across the chest, and if the fabric has no weight to it, it just won't.
The gap between a good one and a sad one shows up fastest at the back, where the lining usually is, and in the buckle that sits at the base of the spine. A proper waistcoat has a front in wool or a decent wool mix with enough body to hold its shape when you sit down, and a lighter lining fabric at the back that lets it move with you instead of pulling. The buttons should feel like they belong to the thing, not like they were posted on afterwards, and the buttonholes should be bound neatly rather than left with thread fraying at the edges after two wears. Darts at the front matter more than people expect, they're what stops the fabric pooling under the bust or gaping at the sides when you reach for something on a high shelf. Cheap ones skip the darts to save on cutting, and you can tell within a day of wearing one.
Where it struggles is anywhere you need real movement or real warmth, because a waistcoat gives you neither. It's not outerwear, whatever the shop mannequins would have you believe, and I've stood at a bus stop in one in October regretting every choice that led me there. It also doesn't love a very soft, unstructured shirt underneath, the two fight for the same silhouette and the waistcoat usually loses, going slightly baggy where it should look sharp. If you're after something that moves freely for a long day on your feet, this isn't it, save it for the parts of the day with less walking in them.
Where it genuinely shines is on days that need a bit more formality without a full jacket on top, an interview, a smarter work event, a winter wedding where a blazer would be too much and a cardigan too little. Autumn and early spring suit it best, cool enough that a sleeveless layer makes sense, not so cold you need the coverage a jacket gives. Try it done up and sitting down before you buy, because a waistcoat that looks sharp standing straight can ride up uncomfortably the second you're in a chair for an hour.