Hmm, well by default admin@system-domain has no vCenter permissions but it should be able to login. Odd that it's saying incorrect user name or password. Maybe try resetting the password on it? Here's the KB on how to do that: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2034608
Also, if you do plan on using that account as an admin in vCenter, check out this guide: vSphere 5.1 Gotcha with Single Sign On (SSO) Long White Virtual Clouds It mentions setting up admin@system-domain permissions.
~Brandit