yes, the key is to always have some sort of migration happening so every problem can be addressed by saying "this will be resolved after the new ERP is rolled out"

And you can decline to do literally any piece of work because you need to focus on the migration.

Perfect scheme.

Or “it’s best to wait till the migration complete before our team needs to do anything”

The latter half of your sentence suggests this project may actually have been a roaring success (for the vendor).

Depends on the vendor's goals. If it's to fatten and butcher a pig, and never see any future work from them, sure. If the vendor is hoping to have a sustained profitable relationship then not so much.