To be honest I don't think there is a sharp division line between VPS or IaaS. They at least intersect. If you try to differentiate, usually you will find definitions that VPS is more like dedicated server, just that it is virtual. So they have the same pricing model - longer term (at least month), pay in advance, often not automatically provisioned. In IaaS you are are charged per actual use, usually in hour intervals. You have API to automatically create new or shut down instances when needed (scale up and down) ... and theoretically you have infinite computing resources available.