Agreed. If the team sees a reason to sign Vatanen, it should be on top of keeping Manson, not to replace him.
As much as I've always liked Manson, I don't see him as part of the future core and do not like the idea of having to extend him at age 30. Trading him to a contending team in exchange for quality futures makes a lot of sense for a team that won't be contending over the next couple seasons.
We don't need a perfect fit in terms of a stop gap to fill a spot in the top 4. Vats has proven he can handle 20+ mins per night, and to be honest more so than Manson has shown he can shoulder the burden of actually leading one of the top two pairings if/when one of Lindholm/Fowler are out. But again, the idea is to gain young assets and admirably fill the minutes with another top 4 D. Whether that turns out to be a net gain, loss or a wash for the current roster isn't the top priority here but rather asset management.
For sure I wouldn't want to see Manson moved without at least adding some sort of stop gap to help fill out the top 4 for the time being. I do worry about running Gibson into the ground over the next couple years.