From the certification team: "VMware Certification continues to review and where appropriate revise its existing certification programs. The VCP5-IaaS certification currently focuses on vCloud Director v1.5. In the coming months, we will be considering updating this particular certification exam to encompass the technologies and solutions with vCloud Director 5.1. There are no definitive schedules for this update at this time."
Which for you means IaaS will not be updated to 5.1 in 2 weeks.