I think I'd just get rid of Darkvision completely before making it a cantrip. If the goal is to just make it possible for everybody to have it, what's the point of it?
Upon further reflection (and reading comments) I do think my suggestion of differentiating underground vs. outdoor darkvision by the light source (torchlight vs. starlight) for example, while thematically appropriate, is an unnecessary level of complexity, and is just begging for annoying edge cases. (Which is Dancing Lights?). So I think what I would do is just make Darkvision let you treat dim light as bright light, across the board.
Tables that don't pay attention to the radius of light sources, or the difference between bright and dim light, wouldn't see any benefit, but they're already ignoring some of the rules anyway. I'm guessing those same tables also aren't imposing disadvantage to Perception checks that rely on sight when characters are using Darkvision.
My sense is that a lot of people like Darkvision because it lets everybody stop worrying about light sources, including the issue of having a free hand to hold a lantern or torch.* (In a campaign I'm playing in now I paid the 50g to have continual flame cast on my sword.) But I think the lighting rules in the game add depth and tactical complexity, if you take the time to incorporate them.
One thing I like about VTTs is that, used correctly, they force you to think about it. When a player says, "I put the lantern down and draw my sword!" the DM can remove the light source from the character and make it stationary on the map. It really does add a whole new dimension to combats.
*DMs who feel this way can just decide that their dungeons have ambient lighting...torch sconces, glowing fungi, whatever.
