NSX has the native ability for IPSec VPN failover. VMware Cloud Director has no method to manage or support it.
VMware Cloud Director 10.6.0.1
As of VMware Cloud Director 10.6.0.1 and prior IPSec failover is not supported through NSX-T.
Implementation may be worked around by directly implementing the failover in NSX. The implementation would have to be managed and supported outside of VMware Cloud Director.