VeganGains fails to understand Peterson's theology: that Christianity is a defacto accumulation of the 'fittest' moral principles throughout human history. One of these moral principles is predation, which even Vegans ascribe to in their personal lives (simply participating in a Capitalist economy involves some level predation and therefore suffering).
However, Christianity should be respected (if not swallowed) because it's the only moral system (in the western tradition) which at least mitigates unbridled predation via the innovation of Jesus' Crucifixion; what really dies on the cross is the excuse that acts of God are beyond human intervention...God (as Jesus) is dead, and now humans are alone and responsible for their own actions without making excuses for their behaviour by saying it is 'God's Will'. This innovation opens the door to the development of our empathic sensibilities, which Veganism is built on.
The argument for Veganism (and I am one) is nonetheless strong; as VeganGains mentions they are mostly health-related and Peterson is wrong to stereotype all Vegans/Vegos as religious zealots, however low neuronal health via lack of protein can and often does lead to depression and magical thinking - certainly a risk if veganism isn't done with careful attention to detail.
I just think if we want to promote Veganism then the bleeding heart empathic argument (ie animal suffering) won't work because it's hypocritical. We live in a world which is full of hierarchical structures: VeganGains' own success as a youtube star relies on a hierarchical structure in the form of likes and views.