I agree with srg36 and hc255 view that they are important parts of a project but may not necessarily be the absolute most important. They are only one part of a large process when developing a project. Still, the items you mentioned information distribution, solicitation, source selection, and managing vendor relations are important but to get there the team had to get through the previous phases before then. Every step in a project from start to finish can have a big impact on the project so each step should be treated as the most important as you go through them. If the planning phase is executed poorly then it may negatively affect the upcoming steps in the project. So every step should be treated as the most important.