You would try to force a redundancy switchover so the actual VSF Member with the Commander role will reboot to let the other VSF Member with Standby role to take the Commander role as a result of the switchover...once in this new status then reload that new Commander...but, as you understood, the redudancy switchover approach is equivalent to the reload approach in case of a two members VSF deployment (and Aruba 5400R zl2 is necessarily a two-member VSF stack).
IIRC probably, since you're trying to operate on the actual VSF Standby member and not on the actual VSF Commander, a vsf member 2 shutdown command would work without causing any VSF Split scenario because, if that command is accepted, the VSF Standby member with VSF Id = 2 will power off and it will be there just one active fragment made of your VSF Commander (you have VSF Split mitigation mechanism in operation, isn't it?). Powering off is not like just rebooting...but, coupled with a cold boot, it could be used to simulate a warm reboot.