How to take the backup of vRA managed VMs?
Back them up just like you would any old VM. They're still VMs.
and then import them in vRA Again?
As long as the VM id is the same, there's no need to import.
During VM deployment process in cloud, users in the VRA self-service portal, we can choose to protect machines with a predefined backup service level.
It will automatically adds data protection backup to their virtual machine, but also enables them to initiate on-demand, point-in-time backups and restores of their virtual machines.