If you are aiming to make a high qaulity 2D game, I don't think you should be scared of 100mb of VRAM. Steam's survey [>] is probably an okay way to see what a lot of PC gamers are using, only about 7% have 128mb or less VRAM. So unless you're making one of those mythical games that everyone can run (or a game for non-gamers), I think you'll be fine using 100mb or so of VRAM.
If you make levels as a single large graphic, you'll end up with a single level consuming all your memory budget and leaving little space for other objects. Also it'll look really static.
I'll just quote the Wii's memory limit (88MB) and that memory is SHARED (both textures and code and data). Then I'll link to Muramasa.
Your best bet for high quality 2D games (and any other high quality games) is NOT disregarding limitations, it's finding workarounds to them. It's SMOKE AND MIRRORS.
Draw your art in layers.find parts that repeat. Use only one graphic for each. You have a huge tree? ok, do you really need each part of the trunk to be unique? no. Use a trunk section and place copies in Construct. Place branches on top of that. You just saved 50MB of VRAM.
tlr;dr..... draw the basic blocks of your art in another software, assemble it into it's final form in Construct.