Than its easy.
1. Create a new Portgroup with VLAN and name ist MGMT/vSphere
2. Now you can connect with your Browser to the ESXi Hostclient and modify the vNetwork of your vCenter VM and move it into the new Portgroup
3. Open a ESXi console (iDRAC/ILO) and modify the Management network settings. You need to modify the VLAN
Thats it.
Regards,
Joerg