Pressure vessel nozzle flanges are with the 300# ratings as per the given process data-sheets. Can we use the 150# rating standard body flange for this pressure vessel ?

Question is not clear...But one advice don't change process data sheet nozzle flange rating unless it fail.

question is this : If the all the nozzles of the vessels are with 300#. This is the min. requirement of the specifications. If any one want to use its girth flange(which is also as per asme 16.5) with the 150#, is it acceptable, if it complies ASME 16.5 pr-temp ratings ?

Yes it is technically acceptable if it is satisfied with ASME B16.5 requirement as it is body flange and is not related with any process connection or instrument connection.

But check who is supplying the mating flange/mating blind flange for the same. If it is in your scope then it is OK but if it is in client's scope then you have to ask him also.

Also final decision is in client's hand. But technically it is accepted.