How easy is finding related vanity addresses? Is it easy to find out related vanity address '1googlex' if we already know '1google' vanity address pair, than through usual vanitygen iteration process? if yes, can this be extended further, like, '1googlexx', without exponentially increasing difficulty?
Zero. They are completely unrelated.

And if you think about that, it is quite obvious... otherwise it would be very easy to steal known addresses: you would just have to find a similar one.