Thank you for your kind words, its good to see this site is providing valuable knowlegde for our community.
First of all to answer your question, has this software being configured or are you going to use it straight out of the box.
In my experience alot of software like this usually has some configuration involved and as a result would fall into Cat 4 in terms of GAMP.
If this is the case I would push for a full lifecycle of documentation.
This should not be a big issue as you have said that the company already has a design and functional spec. I would certainly use this documentation when going throught the life cycle, it would certainly cut down on the amount of paper work you have to develop.
Another point of interest before you begin, did you audit the vendor before purchasing the software?
Actually there is no venodr audit or vendor assessment for the present software.
We are doing retrospective validation for the existing software. When we perform vedor audit that is shuould be at the time of purchasing or at the time of validation.
How can we audit the vendor for retrospective validation.
It should have been performed at the time before purchasing to see if the vendor was up to date with current practices and that all your requirements were met.
You could retrospectively audit the vendor now if you have the time. It maybe worth while if you can get your hands on a design and functional spec. These are documents that the vendor should have and would save you time having to create them.
If not dont worry as the system is up and running now it should be relatively easy develop design and functional specs as the software has been functional for a while.