So I can't use this mod on a server that doesn't have it as well? If the server has the mods then it would cause extra work for others to join if they don't have the same amount of VRAM?

I haven't really looked into this, but the wiki says this:
"All game instances need the installation of exactly the same game-versions and mods."
So I would assume that it is no different with your own server.

Regarding your second question: I would assume that since the game is rendered locally, the individual computer must fulfil the VRAM requirements.
I would suggest you start with the weakest system and use the version of my mod where the dependency are all optional (Factorio HD Age Modpack (all optional)), then enable the most important ones first and see how it works. If everything works, you can then gradually add more packages.

when I've attempted this i get very confused. the dedicated server fails to start with the modpack installed (i believe due to it running in headless mode.) but without it in the server, there is no way to join with the mods enabled client side.

hoping there's a fix for this!