VPN enables you to extend your own network across a different medium - one that isn't necessarily secure. You would use VPN for access to your company's internal network from your home, and you would use it to take users securely from one website to another, maintaining confidentiality of data and identity.
Cloud computing enables you to send data into the cloud, probably using VPN technology. I have some doubts about data security in a cloud context, because I'm concerned with who manages the cloud, how I can easily find my data, and who else might have access to it.
What I'm really saying is that it's not easy to make a straight comparison between what are essentially two different things.