In the smaller coffee can, combine half-and-half, sugar, and vanilla; stir well. Put the lid on tightly. Place a thin layer of crushed ice on the bottom of the larger coffee can; sprinkle with 1 tablespoon rock salt. Place the small can inside the large can.
Pack the area between the cans with crushed ice and rock salt using about 2 tablespoons rock salt for every cup of crushed ice. Put lid tightly on the large can.
Roll can back and forth for about 10 to 15 minutes (protect countertop or floor with a towel, or roll on a cement floor). Lift out smaller can and carefully remove lid.
Scrape down sides and stir all the ice cream together. (If the ice cream is too soft, repack large can with fresh ice and rock salt; replace lid. Roll back and forth a few minutes more or until firm.) Makes about five 1/2-cup servings.