I didn't say "humans should never cause animals to suffer."
I said that humans shouldn't
oppress animals,
meaning that we shouldn't treat them as commodities
and we should respect their rights.
Obviously it's impossible to never cause any animal suffer,
just as it's impossible to never cause any human to suffer.
For instance, if you don't donate all of your available organs,
you're causing another human to suffer and die.
We accept that, because we'd rather live in a society where people
have rights over their own bodies, even if it hurts others.
However, just because you can't avoid hurting other humans sometimes,
that doesn't mean that you're justified in enslaving, murdering, and eating humans.
Likewise, it's wrong to do any of those things to animals,
even if human civilization inevitably hurts animals sometimes.
If you want to go from a pure act-utilitarian perspective
(which is psychologically impossible, hence rule-utilitarianism),
then indeed, you would not technically be a vegan,
because you wouldn't hold any special principal regarding the use of animal products.
However, if you were maximizing utility, you still wouldn't eat meat or dairy products,
because those things cause massive suffering to animals,
and cause environmental damage,
with no benefit (except taste and slightly easier access to B12) to humans living in developed economies.