While there is no definitive link between pip-tazo and risk of AKI, there have been 5 different groups, with internal medicine and ICU patients, showing a possible association. To me, this adds one more reason to at least think twice about reflexively ordering pip-tazo for every sick patient.
In other words the evidence favoring the nephrotoxic association is low level but more and more is coming in in bits and pieces, adding to the concerns.
What I get tentatively from the available data, and this is just a hunch:
Vancomycin as nephrotoxic all by itself is a difficult sell for me. Maybe in patients who experience very high trough levels, but I don't know.
Zosyn as monotherapy appears to have nephrotoxic potential.
Vancomycin seems to potentiate the nephrotoxicity of Zosyn.