Graphics support is only a part of the puzzle. Many devs have never used Linux and know nothing about it. And they hate to learn a new operating system just to support 1% of the players. Building a game for Linux is easy if you already know your way around, but otherwise it can be quite complicated. Valve does offer the compatibility layer (set of system libs like SDL, C++ libraries, etc., that are deployed with the Steam client which your game can use) but this means having to build your game in a chroot environment. Once they see all that, many Windows-only devs just run away.

And then, there are runtime considerations like where you store the player data, cache files, and the case-sensitiveness of the filesystems which many Windows developers are completely unaware of.

So no, Vulcan doesn't give you "zero effort" Linux ports.