The big difference between traditional VPS and IaaS services like EC2 (or Google Compute Engine, etc.) is that the latter has dynamic scaling (and a pricing model built around the assumption that you will use dynamic scaling) as a core feature; for traditional VPS-style work on an IaaS, you are likely to end up paying a premium for flexibility you aren't using (and possibly dealing with some attendant management complexity from the same source), but, other than that, IaaS should be a complete substitute for VPS.