Hi.
I do agree with tatakt. In every 1-bit ADC, VLSB = Vref/2 (assuming VFS = Vref and the input range is between 0 and Vref), i.e. if the input is greater than Vref/2 then the output bit would be 1 (logicl, digital output level) and if it is less than Vref/2 then output would be 0.
However, ofcourse in some input-output characteristics we consider this comp. level as VLSB/2 (which is Vref/4 here) to make the mean of quantization error zero.
I think now it is clear that VLSB should be nothing but Vref/2. (and for an N-bit ADC, VLSB = VFS/2^N). just sketch the input-output characteristic again for yourself.
Regards,
EZT