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?
Finding a vanity address is a random process. Even if we find an address with N letters we are interested in, it does not change anything about finding an address of N+1 letters.
Finding an additional letter in a vanity address pattern requires on average performing 58 times as many calculations as it took to find the original length.
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.