Said as someone who has been on both sides of that invoice.
What a vendor cannot do is work out what should be specified, because that requires knowing where the work actually hurts, and that knowledge does not survive a workshop.
What can a vendor not do for you?
What gets described in requirements sessions is how the process is supposed to run. What gets built is that description. The gap between the description and the reality is where adoption dies.
If you are going to keep one capability in the building, keep the one that decides what to build. The rest is negotiable.