My Vancouver-New York leg is on a Lufthansa Round the World Fare business class. My side trip to Boston is with United on economy basis (back to New York by train). I then fly from New York to Europe on Lufthansa.
So what are you booking here?
- YVR-EWR-BOS-open jaw-EWR-Europe single ticket. No problems.
- YVR-EWR-stopover-Europe one ticket, EWR-BOS one way another ticket. I guess this is the one. I will detail this below.
- YVR-EWR-BOS-EWR-Europe and you intend to not fly BOS-EWR for some reason. This is just a side note but it's worth noting: don't do this because the airline will cancel your EWR-Europe and all further legs.
So the question is, does Air Canada interline your bags on two tickets? Some of us have long enough memories to remember the disastrous years of 2006-2009 when the answer was no, but those days are gone. (Maybe Air Canada one day will even be a decent airline, perhaps if the government allows bigger airlines from other countries to compete with it. One can dream.)
According to this 2018 post and the answer to it, AC will now check your bags through to even an AA flight and AA is not even in the same alliance unlike UA.
Also, since you have a business class ticket, Air Canada will actually try to help you. So I wouldn't worry much here. MCT is 40-45 minutes at EWR so 150 minutes is waaaaay enough.