I would say there may be many different differences some good and some that are cons.
I would think a key thing to cloud from vps is that with cloud you have your service across a platform on many other nodes make it so that if one fails there will be a turn over and it's less of a chance that your site will be down.
With cloud over vps it's very easy to add more resources to your server(s) without any downtime
Cloud is a buzz word currently. everything is a cloud now and everything is hosted in a cloud. You should compare some concrete implementation of a cloud with VPS. As for me main idea is that cloud allows you to pay for resources that you actually consume. Not like with VPS where you pay for resources that you potentially can consume. But this doesn't mean that cloud will be cheaper at the end.