In a previous role, I helped work research, purchase, and implement a call center scheduling "Workforce Management" (WFM) solution. WFM is software that helps call centers schedule their agents based on historical data as well as monitor real-time adherence to the schedules. Like other software that brands itself "enterprise" it includes integration with other systems, key-performance indicator (KPI) reporting, and dashboards. The particular package from Verint systems included shift-bidding and ELearning features.
Playing the role of cross-functional (weak matrix) project manager on the client-side, we compared vendors, used Gartner research, did several demos, and completed procurement (including the paperwork trifecta of order form, master services agreement, and statement of work or SOW) over a few months.
Here are a few things that validated concepts I've learned from PMBOK, school, and personal experience with other software purchasing situations.
During research, it's important to focus on the business needs over technical benefits, but make sure the solution is supportable and approved by IT.
A vendor may present a list of deliverables (SOW), but keep your own milestones, schedule, and success criteria. Though supported externally, it's still your project so it must meet your own objectives.
Technical issues need to be addressed and prioritized with the business owners, technical team, and third-party vendors. Large issue affect multiple stakeholders differently. When in doubt, ask!
Managing expectations and getting the right (amount of) information to the right people is a large part of the project management and purchase evaluation process. I don't envy line managers in the amount of data they have to handle.
Try to separate the technical issues from project progress; but don't hesitate to hold your vendors ("sellers" in PMBOK parlance) accountable to project goals.
Above-all, a positive focus on how to make the project work has seemed to be more productive than keeping tally only to blame someone later. Definitely track issues, but approach them with the vendor and other parties cooperatively.
Whatever the service or software, it makes sense to focus on the mission and work with the other team while still holding your partner accountable.
Good luck with your next implementation!