From what you write, I assume that you 've installed vCenter 6.0 Update 3, which is the first version that doesn't support ESXi 5.1 anymore.
Unless you already have ESXi 6.0U3 hosts in the environment, it might be an option to setup vCenter 6.0 U2, add the hosts, do the migration, and once the ESXi 5.1 hosts are removed again upgrade to the newer version.
Other than this you can only shutdown the VMs on the old ESXi 5.1 hosts, remove them from the inventory, add them to the new hosts' inventory and power them on again. This of course assumes that all VMs are stored on shared storage.
André
PS: see VMware Product Interoperability Matrices