In all the upgrade threads out there, the conventional wisdom is to delete the file called "vendor.xml."
What about this scenario: If my 8900 is Rogers-branded Blackberry but I use it in the US with T-mobile's service, and I upgrade it with a T-mobile upgrade, should I keep the file vendor.xml?

The vendor.xml file has no bearing on your chosen carrier... ONLY when loading an OS onto the device from another carrier other than the branded carrier.

If you are loading a Tmobile OS on to a Rogers branded device, you need to delete the vendor file. Otherwise, the OS may not even appear as an upgrade when opening DM.