In my experience, VH4 has a tad harder bite, stronger midrange and tighter bottom. Einstein is almost the way between the VH4 and Herbert channel 2 + without midcut.
The Einstein Mega-mode has a bit rounder mids, stronger growl. It is also less agressive in terms bite and attack. It chuggs wondefully and the lead tone is soulful. Thrash riffs are not a problem. If you don't down tune lower than C and play too fast on very low notes, you will be fine.
I still would like to add that, the "tone" is still similar as Mo stated. The feeling is different and they sit differently in the mix. But Diezel is a Diezel.