They are somewhat related but still quite different:
VLSM allows a single major (i.e. classful) network to be subnetted using multiple subnet masks.
CIDR allows summarisation at any bit boundary, even if it results in a large network than is possible using classful networks. For example, you could summarise networks 172.16.0.0/16 and 172.17.0.0/16 into 172.16.0.0/15.
Paresh
