the imc isnt powered by vdimm afaik, but it needs vdimm since the memory bus is using it, and its interfacing with that bus.
so some of the logic obviously is exposed to vdimm, i dont know how exactly it works, but this logic can work fine with a higher signalling voltage on one side compared to the other, ie higher vdimm than vtt or higher vtt than vdimm, but if the difference is too high, the logic will degrade and at some point burn out.
the ideal situation is where vdimm=vtt i guess, but it really doesnt hurt having higher vdimm than vtt, as long as its not too high.
intels default vtt is around 1.1, not sure on the specs, but they accept lower than this, so lets say 1.05 or 1.00 is still acceptable.
and for vdimm they say more than 1.65v is not ok, again probably 1.75 is still acceptable, so that means a voltage diference of up to 750mv is probably acceptable, which means 1.1 vtt is ok to run 1.85v vdimm, and for 2.3v vdimm 1.55v vtt would actually be enough. then again thats pushing the limits, and more importantly, it really doesnt make sense to run that high vdimm with that low vtt... if you run high vdimm its cause your tweaking your mem to get a tiny bit of extra performance, and if your doing that, your more than likely to be tweaking uncore as well to clock it as high as possible, which means youll be running vtt at 1.55v at least.

if you look at the usual combinations of vdimm and vtt people are running youll notice that nobody is running them more than .5v apart, even if they dont care about the voltage diferential. thats probably why we havent seen more than a handful of i7 chips go bust from this, at least that we know of... and even those might just be freak accidents and not really caused by vdimm and vtt being too far off from each other.

BUT, the newer chips requiring less and less vtt to clock high with might be a reason to worry, but actually the newer memory all needs less vdimm to max out as well, so then again things seem fine... combining d0 or future chips with memory that needs 2.3v to max out might be problematic... but even that is unlikely to cause problems as barely anybody would run that high vdimm 24/7