No, not possible. You have to clear Canada Customs, and then US Customs pre-clearance. It is not possible to stay airside. There is no streamlined US to US connection capability at YVR whatsoever.

Keep in mind that AC18xx segments are on the A319 "Rouge" service which may be quite a bit less than desirable for humane mileage runs.

IMHO, even 1 hour would probably be unrealistic for UA. I'd say you would need 90 minutes, on an ideal day, to make it work.