Why not.
Sure it would be better to buy a jacket made by a vegan company. but if I really liked it, I would buy it.
I buy my vegetables at a grocery store that sells meat. My shoes come from a company that also uses leather in its other shoes.
I even once went to Taco Bell cause they were promoting vegan menu options. The world isn't going vegan in a day. So let's take it step- by -step.
In fact, you could even argue that it's a vegan's duty to buy stuff like that. If they don't sell it they will stop making it. And in the meantime, as long as its available, nonvegans, almost vegans, proto-vegans might buy it, too. Then the company will think they have something and make even more. And make less goose down filled jackets.
The first sleeping bag I bought for backpacking was goose down. That was like 40 years ago. But then ten years later I got a synthetic filled bag from a local company (Snow Lion). I can't remember all the details but I think the synthetic bag was better than the goose down bag in almost every category.