It all depends..
If your target player has a weak old GPU then 15 mb is what u need. Also there is nothing wrong in getting this limit upper. Speaking for myself: in my project I use about 220-240 mb per layout (of course i'm trying to get this lower optimising the game but still) With that amount of VRAM occupied I haven't any issues when running demos on my laptop (256mb VRAM). Everything works fine and smooth.
You can also optimise your game and gain some spare VRAM for the new graphics data turning off blur effects / using pixel shader 0.0 / turning on "point'" type of interpolation in your game. Of course if those effects are not a big deal for you.
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Anyway I think only testing the game can tell you whether to increase the VRAM usage limit or leave it as it is.