True, the Vault has far more content, including new releases. Very little unique content on Steam, either, so Vault tends to be the one-stop shop.

I put my stuff on both, though, because I guess some players are used to looking on Steam.

The one-touch Steam install is nice, but there again the Vault has NIT which is similar.

For some reason, Steam Workshop mods only work if Steam is running, whereas the Vault install method is DRM-free.

Those who prefer old-school manuals might like this:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1309774090

Why use old ways if you can reach more players and easy of use with workshop? Vault was good 20 years ago but steam workshop you click one button and everything is installed automatically for even then newbiest player out there with no copy pasting right folders and know hows.

Well, since you ask, there are at least two reasons for using the Vault.

Firstly, Steam Workshop is not available to players who buy the game on GOG, so it actually reaches fewer players.

Secondly, the Vault has vastly more mods than Steam (many thousands) so it is the go-to for serious players. Most active authors are publishing exclusively on the Vault, Steam being unpopular for a host of reasons, not to mention the enormous back catalog.

The Vault is not the old IGN site of 20 years ago. NIT provides one-click install.

Arguably, Steam is less easy to use. There have been many reports of issues that are only resolved by moving the files manually from Workshop folders to Documents, though that might be due to the DRM issue I mentioned above.

For some people - especially mod creators - it is important that the Vault is run by fans for fans. The history is that IGN shut down the old Vault for commercial reasons, but fans were able to rescue the mods from oblivion. We can debate whether Workshop or Vault is most at risk of closure, but that's one reason why the Vault inspires loyalty.

Having said that, it's not either-or, which is why I publish on both sites.