I too am planning a trip from the USA over to Vladivostock (possibly via Japan, if I can afford it) then through Mongolia, Kazakhstan, Russia, and back to the UK...
I hope to ship my bike from the UK (I live in London) to the US in May, and am looking into then shipping the bike from the US to Russia (or Japan). When are you thinking of going? Would be good to at least meet up and perhaps ride at least part of the trip...
I ride a KTM 990 Adventure. I've ridden in Siberia before - but only in the western bit - around Ekaterinburg.