Since veganism is a social justice issue, which by definition is opposed to exploitation and oppression in all forms, most vegans are also for human rights. Animal rights is an extension of these principles. It is just a shame that it doesn't work the other way around - most people who advocate for human rights oppress and exploit nonhumans. Discriminating against someone because of their species (speciesism) is no different, and no more justifiable in principle, than discriminating against someone because of their race (racism) sex (sexism) or physical/mental abilities (ableism). I recommend reading some of Aph Ko's work for more understanding of these fundamentals