Translation:We have different definitions of what is fair.
I think it works this way... if vad introduces a subordinate clause and there is no other subject in that clause it must be followed by som . Same thing with vem and vilken.
Hon undrade vad som var bäst...She wondered what was best.
Hon undrade vad han ville... She wondered what he wanted.
Han frågade vem som ringde...He asked who called.
Han frågade vem hon ringde...He asked whom she called.
I read in an old learn Swedish book that “ if vem, vilken or vad introduce a subordinate clause, and there is no other subject in that clause, they must be followed by som:
Han frågade vem som ringde. He asked who phoned.
Hon undrade vad som var bäst. She wondered what was best.
Han frågade vem hom ringde till. He asked whom she phoned.
Hon undrade vad han ville. She wondered what he wanted
The question had already been answered above three years ago.
No, you need the som. I think the construction is similar to in English "that which", or less directly "what it is that is" instead of just what.
I.e. We have different definitions of that which is fair.
Vad & som seems to fairly nicely correspond with that & which respectively.