vCenter has to be first because you cannot manage ESXi 5.1 hosts with any other version of vCenter except 5.1 (and higher).
Like Aravinds said there are a lot of other components involved with vCenter 5.1 that need to be installed and configured.
Focus on getting vCenter 5.1 up first. Then you can worry about the small things like VUM and the actual ESXi hosts.
I say small things because upgrading ESXi is trivial at best.