Since you mentioned, you were able to logon using vSphere thick client - I believe your vCenter is all fine.
Some questions/steps -
1. I believe you have configured everything using FQDN. Do a nslookup of the IP/FQDN(of vCenter Server) from SSO server and make sure you get a reply correctly.
2. Ensure there is no firewall between vCenter and SSO.
3. If the nslookup is not working, ensure all FQDN entries are recorded in DNS.
4. If you want a short-cut, manually keyin the entries of vCenter in SSO server's host file and SSO server's in vCenter.
Thanks,
Karthik Varadaraja