In our own experience with client implementaions we have seen clients under the impression that migrating between applications is as simple as an excel export and its subsequent upload into their new application. A very common misunderstaning
of just what is involved in getting an new application up and running is the major cause of missed goals.
Under-resourcing the implementation is a major point of failure, often caused by underestimating the overall scope of an implementation of new systems, and the mis-judgement of available time of existing employees being able to carry out the day to day responsibilities as well as being focused on the implementation.
Often we have see organisations that implement payroll systems and only when it is close to running live do they start to give some thought as to the outputs they require. Outputs should be documented as part of the project plan. If other departments need reports or you need interfaces to other business systems, make sure you identify these requirements up front and integrate them into the project plan and budget. Failing to do so will very quickly cause cost overruns and ferment the lack of support from internal users.
In short, an installation is nothing more than actually configuring the software for your company parameters and leaving you the customer to figure out where to go from there. It would be a correct assumption to say that the installation is a simple process, while the implementation really is a more complex undertaking and commitment, bringing with it a host of inter-related tasks that need to be completed in order for the plan to succeed.
Your vendor cannot complete the implementation for you, because your vendor does not understand your payroll requirements, your vendor cannot confirm that the payrol details rolling out of their system is correct or not. And, this is not that their application does not work, it is not that they do not know what factors are involved in the calculation of that particular pay, only you can ever sign off on that result.