The problem you’re describing is extremely niche. A rescued cow that is still lactating, where the calf could not be rescued, will likely need to be milked in order not to develop any issues in her udder. That milk is a byproduct of a medical procedure made necessary by the abuse she suffered. I would see to it that it gets put to use for the cow children first but in the end whether you pour it down a drain or your throat makes no difference. However that milk will dry up after a couple months anyway. No mammal produces milk just because, it is always to provide for a child.

As for if making cheese out of that milk would be vegan, it depends on the rennet I suppose. For cows you don’t have to butcher calves anymore so it’s likely gonna be possible. For goats I don’t know.

A rescued sheep might need to get shorn regularly due to a problem of its breed. However again the focus is not getting wool, it’s to help the sheep, which means it should be shorn more often than at a farm and not as close to the skin than “normally”, resulting in a poorer wool quality since the individual hairs are much shorter.

Like you could ask if eating roadkill is vegan. Sure, if it doesn’t interrupt any animals grieving process or rituals, whatever, who cares. I think our ancestors probably scavenged a lot of the animal products they used.

But there is an ethical issue with breeds that depend on humans for their survival, it’s that the people hold the power of life or death over them. There can be no meaningful consent there. Additionally they have to be held in captivity if they would die in the wild. Which is why rescued animals usually aren’t allowed to breed. They could never be allowed to breed as they wish anyway, otherwise their population might overwhelm the care humans are capable of giving them anyway, so let’s cut it off entirely.

So your hypothetical vegan animal products depend on rescue farms that would be phased out. There is no vegan way to get animal products in perpetuity.