Accessing the Placement from the Vacancy is easily done in Mercury.
Click the Vacancies focus and locate your Vacancy > click the Name to access the Vacancy.
Click Related > Placements
You'll then be presented with the associated Placements.
Putting a check in the circle and then clicking edit will open the Placement, as well as double clicking in the area shown.
You are now in the Placement. If you need to quickly navigate back to the Vacancy, click on the Vacancy name and you will return to the General tab of the Vacancy.