The Cascades is a wholly Amtrak service that VIA has nothing to do with, other than provide station services in Vancouver. The only joint Amtrak/VIA operation is the Maple Leaf, VIA 97-98/Amtrak 63-64.
The cafe service on the cross border services varies a lot, and I don't think you can project one onto another. On the Maple Leaf, the cafe service is provided by VIA with a VIA attendant and a VIA menu. The get on and off in Niagara Falls, ON and usually open/close around St. Catherines. On the Cascades, it is a wholly Amtrak operation. The cafe closes briefly northbound at Bellingham so the conductors can do one last ID check, and southbound at Blaine for CBP Customs inspection. They provide full service. One idiosyncrasy is there is an (apparently somewhat informal) deal with the BC Liquor board that as long as they're enroute, the train is kind of considered American and the Liquor agency ignores its presence. But, while the cafe is open during boarding at Vancouver, they do not sell any alchoholic beverages. As soon as the wheels start rolling, alcohol is for sale.
I ride the Cascades a lot and the Maple Leaf at least annually and am quite familiar with current practices on both. The last time I rode the Adirondack, though, was several years before COVID. I seem to have a dim memory that the cafe wasn't open southbound out of Montreal until after the US border, but I am really unsure and in any case it was a long time ago.