After a little bit of pain I am able to assign tags to vRA-provisioned virtual machines on vCenter through vRO.
have a look into workflow.
1) Configure VAPI endpoint in vRO.
2) Create property definitions (vRA.Deployment.Tags) in vRA.
3) Create a subscription Event Machine Provisioning as below.
4) Edit Blueprint and Add custom property Extensibility.Lifecycle.Properties.VMPSMasterWorkflow32.MachineProvisioned
Inputs : payload