Answer
The speed of the Volkswagen should be 2 mph
Work Step by Step
In order for both cars to come to a halt, the magnitude of the Volkwagen's momentum $p_v$ should be equal to the magnitude of the Cadillac's momentum $p_c$. We can equate the magnitudes of the momenta to find the required speed $v$ of the Volkswagen.
$p_v = p_c$
$(1000~kg)~v = (2000~kg)(1~mph)$
$v = 2~mph$
The speed of the Volkswagen should be 2 mph.
