Should vacuum sealed food be exposed to air when thawing? Must food packaged and vacuum sealed be exposed to air while thawing? 

I seal chicken or fish in a vacuum bag. I freeze the same. I transfer to the refrigerator to thaw with the expectation of cooking sous vide. Should I cut the vacuum bag while thawing and then reseal after thawed? 

My fish monger's policy always is to thaw so the product is exposed to air, saying that salmonella grows only in a vacuum.
Members of the Genus Salmonella are facultative anaerobes. That means they'll grow in both air and in what you bag sealer calls a vacuum.*
I'd just leave the bag sealed, unless you want to check it for smell before cooking. That could be informative.



*Not putting down your bag sealer. They just don't pull a lot of vacuum. You've probably got half a psi or more in the bag when its done, plus air diffuses into even 5mil freezer bags over time.