I was looking at
this from sportrac and only the top 51 salaries count towards the cap. Well for base salary the players at 51 and later are making 570k or less, and only a few of those will count towards the cap.
The estimated contract for Mattison is 685k so still above this threshold of 570k
If the Vikings picked at 81 the projected salary for the Lions pick is 730k so perhaps harder to get under the top 51 salaries.
There are players who the Vikings are paying more than the projected salary of Mattison however, who could be cut with very little impact to the team like Dakota Dozier for example.
The easiest way to get out of cap hell would be to just cut or trade Rudolph. But Rudolph is welcoming Bradbury to the team, face of franchise ect. so that seems awkward.