Vibration limits are specific to particular machine and measurement types. Limits for relative displacement on high speed compressors will not apply to casing velocity measurements on slow speed pumps. If you are more specific in your application, I am sure you will get more input.
Here are two good references:
"Machinery Malfunction Diagnosis and Correction" by Robert C. Eisenmann, Sr and Robert C. Eisenmen, Jr
"Mechanical Vibrations" by JP Den Hartog
Not necessarily in that order
After changing the element twice, it would almost be safe to assume that the shaft/impellers weren't the problem. Notice I said almost. I would be looking at the alignment based on the high vibrations and the subsequent coupling failure. The only thing that I could imagine might have changed...