What you will lose depends on what you have in vCenter.
Templates, Folders, Cluster configurations, HA/DRS configurations, Alarm settings, cluster resource pools,
datastore clusters, storage profiles, host profiles, event history,
If you use vNetwork distributed switch,
vCenter Distributed switch configurations will not exist in the new installation and
need to be rebuilt before re-adding hosts; you will probably need to convert those back
to standard vSS on the hosts temporarily before adding them to the new vCenter..
Basically... you lose all configuration except the configuration that is stored locally on the hosts.
That is, basically the list of VMs and the stuff you see in the 'configuration' tab when you click on
a host.
You will need to re-add the hosts as one of the steps in installing the new vC.
You will have the VMs registered on the host, and any resource pools that existed
on the host at the time vC was lost.
However, when you move the hosts into a new cluster, the 'host resource pools'
will be removed.