When you are combining the resources of an external team with those of your in-house SEO team, there is a need to determine who does what, who decides what is to be done, and so on. It is best to have this all clearly defined up front, or else there will be conflicts and confusion, which will slow down the process of increasing your organic search traffic. Be sure to talk to the website program or project manager, not only upper management. In the end, it will be up to the program or project manager(s) to fit the changes into the schedule. Get their buy-in from the beginning. You may want to even involve them in the vendor selection process, because they will have to work with the vendor as well.