I’m not sure that you can compare the two ships. I’ve not sailed on the Vista but have sailed on the Riviera a few times and on both the Sirena and Nautica several times and the experiences are quite different. The Insignia is old, the Vista is new. The Insigia is small, the Vista is a lot larger. I’ve enjoyed all my trips with Oceania, apart from a B2B in the Caribbean when the second, shorter 7 day leg, was a bit rowdy. Also you can’t compare longer voyages to the shorter ones they have a different atmosphere.
Enjoy your cruise but if your Insignia experience isn’t 100% ( I’m not saying it won’t be great) don’t let it put you off the Vista.