var_1 <- list(1, 2)
var_2 <- list(3, 4)

output <- do.call(
    rbind,
    mapply(
      function(x, y) {return(data.frame(x, y, x+y))},
      var_1,
      var_2
    )
  )

output <- do.call(
    rbind,
    mapply(
      function(x, y) {return(data.frame(x, y, x+y))},
      var_1,
      var_2, SIMPLIFY=FALSE
    )
  )

output <- do.call(
    rbind,
    Map(
      function(x, y) {return(data.frame(x, y, x+y))},
      var_1,
      var_2
    )
  )

do.call(rbind,Map(function(x,y) data.frame(x,y, x+y), var_1, var_2))