(lambda args body) is not the same as ($vau args env body), because the latter doesn't evaluate its operands.

Lambda in Kernel is defined as follows:

  ($define! $lambda
     ($vau (formals . body) env
        (wrap (eval (list* $vau formals #ignore body)
                    env))))

When you say ($lambda (x) x), a new fexpr (that ignores the environment in which it is called, because lambdas don't need it) is constructed -- ($vau (x) #ignore x) -- and then it is wrapped, so that its operands are evaluated.

So,

  (lambda args body)

can be expressed as

  (wrap ($vau args #ignore body))