I whole heartedly agree the pressure from vendors can be exasperating. I was thinking if there is a way to turn this around. What if as an example, your IT product vendor were told that they would have to provide expertise (at no cost) to solve current interoperability issues or resolve performance bottlenecks between their product and one other product while show casing a operations admin and management/ monitoring tool - OAM tool? This would be their elite product engineering or premium consulting service participating on-site mentoring your team and not the "support" organization which has to trouble shoot remotely.
Another example, if this is a package solution vendor with a long deployment cycle then the vendor can be asked to offer (again for free) analysis tools or processes or best practices and other accelerator kits that improve speed to market for their product, with the chance to demonstrate components of their next generation product. Again, the vendor has to part with IP (intellectual property) and address your pain point with the chance to demo a product that would or could be a fit in "your" enterprise.
The result would be the ability to extend or enhance the life of existing investments, while evaluating the vendor product, vendor processes and expertise in a real life scenario. Most importantly however, you are putting their resolve to test as to whether they are able to or willing to be your strategic partner and not just a transactions based product vendor.
Of course, I would be remiss in not stating that VMO, PMO and your legal departments would have to help with insuring this was a fair and scientific discovery process and also, that the licensing models were conducive to both parties.
As always your comments are welcome.