I have used so many SEO vendors over the years and I did find one I like, trust, produce results, and doesn't cost a lot of money. What I have learned is that you should be able to answer these questions:
1. How much actual content will they write?
2. Is the content original and not generated?
3. Create a baseline that is third party so you aren't just relying on their reports? I recommend Organic visits in Google Analytics.
4. Can they create a map of keywords to target each month?
5. Can you see the content or is it hidden? Hidden content is not as good. Plus many vendors say they update pages but you can't track it or it is hard to track.
6. Will the company do onsite and offsite linking?
There are more but this could help.