In order to enable DHCP-relaying for interfaces belonging to Virtual Systems on a VSX-cluster, you have to set the context to the specific virtual system the interface belongs to.

List all Virtual Systems on the VSX-cluster
show virtual-system all

Example: Configuring DHCP relaying for an interface belonging to Virtual-system 3
set virtual-system 3
set bootp interface bond0.18 relay-to 10.10.10.10 on
set bootp interface bond0.18 relay-to 10.10.10.11 on
set bootp interface bond0.18 on

set virtual-system 1
set bootp interface bond0.65 relay-to 10.10.10.10 on
set bootp interface bond0.65 relay-to 10.10.10.11 on
set bootp interface bond0.65 on