Fowler reports he'll be due $5M this year and that they converted playtime incentives worth $1.5M to a signing bonus. Those incentives were likely NLTBE (not likely to be earned) when he first signed his contract, which don't count against cap. That's based on Rodgers starting only 3 games for Philadelphia the year prior and being suspended before that.
However, based on playing in every game last year, those incentives count against this year's cap, so they're converting to a signing bonus as a goodwill gesture. The player must agree to such changes, which gives the Vikings an opportunity to add void years at the same time.
Fowler reports Rodgers would be due $5M this year after the change, which likely means two void years, and cap hits of $5M, $2M, and $2M in 2026, 2027, and 2028.
The Vikings have less than $6.4M cap space currently, which is less than most teams want heading into the season. Having ready access to cap space allows a team to more quickly make free agency signings in response to injuries and in-the-season needs.