How can you give an argument in favour of vegetarianism when you eat meat? Which fallacy is committed in this argument?

The reply attacks the arguer's own conduct rather than the argument, an ad hominem (tu quoque).

A case for vegetarianism is met by pointing out that the speaker himself eats meat.

Whether the speaker eats meat has no bearing on the strength of the case for vegetarianism.

Dismissing a claim by charging the claimant with inconsistency evades the reasoning.

Attacking the person instead of the position is the ad hominem fallacy.