Ah, I found the error with your tip....my ocean shader uses POSITION as an attribute in the VS, and as a varying in the PS.....seems to be the issue.
I've now changed the PS to an attribute, and it fails to compile. If I disable the ocean shader, it runs and loads now.
EDIT: but actually nothing works - the landscape doesn't render at all. Neither of these isssues were in the beta, so I assume it's the vulkan engine that's the issue....but from what I've seen, there no apparent benefit anyway? Memory impact seems bigger and no framerate benefit?