This could be an issue with the game using the wrong video card.
It could also be a non-issue; a number of people (me included) are getting low vram warnings when there doesn't seem to be anything actually wrong. You can go into the game's main config file and turn the warnings off if that seems to be the case for you.
On MacOS, you get there via right-click on the app, select 'show package contents', then navigate to Contents/Resources/Java/data/config/settings.json - open that with a text editor, find the line that says "doRAMandVRAMChecksWhenRunningWithMods":false, and edit that false to true. Make sure you don't delete the comma after it, though.
On other operating systems, the path to that file will presumably be at least similar in structure.