Upvote:5
There should be absolutely no problem with doing what you propose. You are correct that "travelling into a bordering country doesn't count as leaving the US for the purpose of the 90 day stay limit", but it does count as "leaving the US". In particular, since you are flying internationally from Portland to Vancouver, the US CBP will get a record that you have left.
If you ever have any doubt, you can look up your I-94 arrival and departure record online.