Using a vacuum sealer removes all the air from a bag, and heats the edge to seal it closed. The water displacement method involves using a freezer-safe resealable food storage bag and while the bag isn’t fully sealed, slowly lowering the pork shoulder into the water to force the air out. Either method would work fine but for a couple of reasons, I prefer a vacuum sealer. While there isn’t a safety issue if you do remove all of the air with the displacement method, it is challenging to remove all the air. Too much air causes can cause off putting flavors or spoilage. At higher temperatures or longer cook lengths, the seals on the sides and bottom of a freezer-safe resealable food storage bag could possibly fail, causing a mess and ruining the cook if left unattended.