I suppose you have answered your own question.
If the job descriptions are too vague, who is responsible to change them. This seems to be a poor case gone bad to worse. Look at the situation here. Markets, economies are changing everyday. This in turn changes employees job roles and their productivity too.
If you have not set the right expectations by communicating a change in job role/descriptions, you surely cannot make a comment on their performance.
The Training Specialist.
6th October 2013 From India, Mumbai
In my point of view when a performance management can be successful, when the criteria of the job would be clear for both employee and employer, thus we have the word in performance management (SMART) which clearly mention the criteria when we are setting the objective for the employee. In my point of view for the current stage you can set the objective based on the Key result area of the job scope, and simultaneously start the process of job analysis for your organization.
8th October 2013 From Singapore, Singapore