Since Vmware vSphere 6 is released the vCenter Virtual appliance (VCVA) has the same specifications as the Vmware vCenter installed on windows (VCS). So you can install the VC virtual appliance without to worry about limitations and also one windows server license less to buy. Downside is that there is nog migration path from VCS to VCVA, so before you upgrade to vSphere 6 you can first migrate to a VCVA 5.5 and the upgrade your virtual appliance to vSphere 6. there is a cool tool do this from Vmware fling website
“The VCS to VCVA Converter Appliance is the winning idea from the 2013 Fling Contest. It allows customers to migrate from Windows vCenter Server with an External Microsoft SQL Server Database to the vCenter Server Appliance with an embedded vPostgres database. The Fling migrates the vCenter database, roles, permissions, privileges, certificates and inventory service. The target appliance will run at the same IP address as the source vCenter.”
Check out the requirements first because vSphere with integrated database is not supported and you will need to install the same version for VCVA as the VCS. On the site is also a video install instruction to follow and prepare yourself
good luck if your trying it.