Not really my experience.
We had already taken 3 Vantage trips before COVID hit. We had a Tulips cruise booked for March 2020, which we had to change. We chose a different itinerary since we had to pick one in 2020 to change to. Later we were able to change that to the tulips cruise in 2021. Well, obviously, that one didn't happen, either, but we moved it to 2022 and it went off then.
I will agree actually getting hold of a real person (that can actually help you) on the phone is a challenge, and can take time, but I've always been able to get things done eventually.
We've always (except for the initial change on our 2020 cruise) been notified of changes via email, as well as the notice section on our online reservation page.
As to your "All communications with Vantage staff start from the premise that they are in the right and you are the problem." comment I've never encountered that.