/usr/sbin/vserver SERVER restart
But it can't be executed from inside the virtual server. To solve this the rebootmgr (robot manager) was created. It is a service which installs a unix domain socket in each virtual server (/dev/reboot). The /sbin/vreboot utility is used to request a reboot from any virtual server. The reboot manager knows which vserver is sending the request and execute the command above.
The /sbin/vhalt utility is supplied as well, which simply end a virtual server activity.
The rebootmgr is a service and is off by default. You must enable it.