It's fairly common for the core to be one vendor and the RAN to have more than one other vendor - IOT (Inter-Operator Testing) takes care of that, and it's in the vendors interest to do this properly even if they'd like to have the complete network footprint as a sale.
What normally happens in the RAN is that an area is defined as "here be Ericsson", and that will be their footprint. Messing around with that paradigm is folly, as the performance statistics of one vendor will not necessarily equate to those of another vendor, therefore you'd need to expertly merge that data to properly understand what your network is doing in an area with multiple vendors.
There are some standards defined performance metrics (TS 32.403. IIRC - that might be the UMTS one, anyway), but they're possibly not the most informative stats for network diagnostics. It would probably be quite hard for a RAN engineer to trace issues as the user(s) experiencing those issues move between vendor sites.