At one time this was a huge blocker for our productivity. Access to a reliable test environment was only possible by way of a specific customer's production environment. The vendor does maintain a shared 3rd party integration test system, but its so far away from a realistic customer configuration that any result from that environment is more distracting than helpful.
In order to get this sort of thing out of the way, we wrote a simulator for the vendor's system which approximates behavior across 3-4 of our customer's live configurations. Its a totally fake piece of shit, but its a consistent one. Our simulated environment testing will get us about 90% of the way there now. There are still things we simply have to test in customer prod though.