In Fast-Moving Environments, You have to Make Your Own Solutions
I realize it's hard to be a vendor to a lot of big companies. Some want stability and verification of easy little point release, and others want to see new features and fixes turned around as quickly as possible. Heck, I run into that every day here. Some groups want stability - which implies very few changes, and others want to see new things they've asked for as soon as possible - even if it means a little loss of stability. I get it... you can't please everyone.
But this can't be an excuse to do nothing. And by nothing I mean you can't have people on site helping to work on your multi-million-dollar product and have them (along with the users) identify bugs and needed features, and then just sit on them. That's not going to be 'OK' with anybody.
But it happens. Can't be helped. If a vendor is unresponsive, and you have to still make it work, you have to exhaust all possible options and avenues before throwing up your hands and saying "Well... we tried everything and still it's no-go. We have to wait for them to fix this."
I've had experience in both cases, and one is not more comforting than the other. You can't stop at the first sign of a vendor problem - you have to inform them, to be sure, but you can't stop there. You have to dig and dig, because you can't be sure that they are even going to look at the problem with the same intensity as you will. Plus, it's another pair of eyes - look and maybe you'll see something that they don't because they're too close to the problem.
At the same time, when you hit the end of the road, you have to accept that it is the end of the road and not try to change facts just because you want them to be different. You have to try and accept the facts. But you can't give up too quickly.
I've run into guys that have simply given up too quickly. Maybe that's all they had... maybe they couldn't see that there was another way to approach the problem. In any case, giving up is not going to make things work. You have to be more determined than that. If the answer can't be found, that's one thing, but you have to be sure that it's not that you're giving up too soon.
After all, you might be dealing with a vendor that's not going to be as responsive as you like, and you may only have a few weeks to get things working or you loose the management backing to continue with the project. You have to take things into your own hands a lot of the time. There's simply no other way to get things working.