When evaluating a professional services company you want to hire, what are the most important criteria you look at to find a good vendor partner? e.g., consulting staff size, references, billing rate, etc.

Reference options for success in the field I am requiring
consulting team sizing
willingness to think outside the box

When it comes to professional services, you want to ensure that the services provided are professional.

Some firms will send their all-stars to the pre-sales calls. But when it comes to doing the work, they will have junior consultants (who they pay less but bill at the same rate, and thusly make much greater margins) do the work.

In the contract, stipulate that the consultants that were on the presales calls are the ones that do the work.  That will not always work. But it will put the professional services firm on notice that you are expecting them not to send the rookies.

 

As simple as this seems I want business outcomes and that is what I am willing to pay for. I like Outcome Based Contracting and the SI to step forward with that confidence and capability to deliver on what I want and they promise.

Number one is will they be around in a year, 18 months, 2 years?  No good if they aren't going to survive.  Number 2 is do they have relevant expertise in our specific area (medical, patient health information.) And do they have the relevant regulatory frameworks in place.
After those, comes references, costs, etc.

At the end of the day, it always comes down to the individuals who get assigned to your account. Take a look at resumes and talk to those folks if you can, on top of some of the other items mentioned in the question.