You can't be 100% vegan, so why try?
The standard definition is 'as far as is possible and practicable', not perfection. A reduction in harm is still a reduction.
Animal products run through modern life in ways no individual can escape: tyres, medicines, building materials, the fertilisers and pest control behind even plant crops. If complete avoidance is impossible, then veganism cannot be the clean moral line it presents itself as, and anyone living it is still complicit in animal harm, just in less visible ways. Drawing the line at diet and clothing then looks arbitrary, and it is fair to ask why that particular line should be treated as a moral obligation when the ideal it rests on can never actually be met.
Traces of animal products are genuinely unavoidable: in tyres, in some medicines, in agricultural inputs. The Vegan Society's definition has acknowledged this since 1979, which is why it is framed around what is possible and practicable rather than around purity.
The reasoning would not be accepted anywhere else. No one argues that because theft cannot be eliminated there is no point having laws against it, or that because some pollution is unavoidable there is no point reducing emissions. The relevant comparison is between the available options, not between an option and perfection.
Rejecting a workable option because it falls short of a perfect one. Also called the perfect solution fallacy. The relevant comparison is with the realistic alternative, not with perfection.
How it applies here: Rejects a substantial reduction in harm because it falls short of eliminating harm. The comparison that matters is with the realistic alternative, not with a perfect outcome.
Replacing someone's actual position with a weaker, distorted version, then refuting that instead. The original argument is left untouched.
How it applies here: Attributes a claim of absolute purity that the standard definition of veganism explicitly declines to make.