In medium bowl, combine cereal and half & half; let stand 20 minutes, stirring occasionally. Strain half & half into a 1-quart recloseable food storage bag; discard cereal. Add sugar and vanilla to half & half; seal bag.
Fill a 1-gallon recloseable food storage bag half full of ice; add rock salt. Place bag with half & half inside bag of ice; seal bag.
Shake vigorously until mixture has turned into ice cream (about 5 minutes). Remove ice cream bag from ice. Serve immediately or freeze until ready to serve.