Becoming a Chinese vassal is better than just working with the United States? If that's the case then they're not worth saving.

They undoubtedly view it as the reverse - asking us why we think becoming a U.S. vassal is better than just working with China. After all, China is far less likely to be able to place China in a "vassal" state than the U.S. can. Canada has learned the lesson that if you give the U.S. what it claims to want in the moment, then the U.S. will return to retrade the deal even further down the road.

No we don't. We want supply chain provenance in North America for critical goods; they're free to make other deals so long as they don't circumvent our trade laws by packaging goods for sale here (as they have done in the past).

If that's all we wanted, there wouldn't have been a need to change existing trade protocols. But what was reported is that we were insisting that rather than enforce existing rules that prevent transshipment, we were instead insisting that Canada have their tariffs and trade policy match our own, exactly, vis-a-vis China. Rather than let them "get the best possible terms for Canadian firms" through trade deals, as you describe it below, we wanted to insist that they erect big tariff walls so that they would end up reliant on the U.S. And that's no longer a tenable position for them.

This is actually true. Why wouldn't we want to get the best possible terms for US firms?

Because the "best possible terms for US firms" in the short run can lead to a worse situation for US firms in the long run. Using Canada's current dependence on US markets to gain a negotiating advantage can yield a short-term win in business terms, but it creates massive incentives for them to diversify their economy away from the US in the longer term so that they no longer face that negotiating weakness. This makes the short-term gains ephemeral and can damage US firms over the long run, since the whole point of dropping tariff barriers between the two countries is to foster a more efficient business environment for firms on both sides of the border.