There are no definitions, so you have to read the context and, yes, that includes interpreting the marketing talk too. But it's not all bs, it's to communicate the general aim of the manufacturer. Generally, when it comes to pickups, it means they copy the specs (and thus sound) of passive pickups made in the 50s-70s. Of course, pickups that are from that period are also considered vintage. I think the specs of vintage pickups are generally established, but not written in stone (largely because pickups made back in the day had very loose tolerances and were all over the place), and pickup makers take certain liberties to differentiate themselves (or improve on designs). Buy yes, more broadly speaking, the vintage sound generally relies heavily on flatwound strings, and basically using whatever equipment was around in the 50s-70s (or their modern equivalents).
But to answer what seems to be your core question: to get more high end, vintage pickups can be a good choice. But that depends on what the baseline is - what pickups do you have currently? Another consideration is also that vintage spec'ed pickups are quite common, not necessarily expensive, and easily found used... so probably a much better choice for a pickup swap than going custom right away, because there's always risk involved, and it may take some trial and error...
There are alternatives to getting more high end out of your bass, some of which are much more affordable and easy to do (depending on your soldering skills). Adjusting pickup height is free, and worth trying. You can try a few different sets of strings, but that also depends on what you're currently using. You can try higher resistance pots, or fewer pots (direct-to-output wiring being the brightest). If you have a pickup wired in series (like a P pickup), wiring it in parallel gives you more high end (you can tone it down with a loading cap, or a few). Or, you can put active electronics in (at least a buffer)...
Also, make sure all the joints and screws are tight, that could kill some high end. And that strings are properly installed, with good break angles and all that. Little things can add up. But ultimately, it could be that the particular pieces your bass is made of just filter out quite a bit of treble, and that's thus the inherent (and unchangeable) sound of your bass.