Considering vMotion network is setup and all the VMs are running on shared storage , If Host is part of DRS enabled cluster :
a) Placing the host is maintenance mode will migrate the VMs (Powered on/off) to other hosts in the cluster , provided the DRS mode is set to automatic. b) Manual migration will be needed if the DRS mode is set to Manual.
Alternatively , you can choose to power-off the VMs (if no production impact) and then place the host in maintenance mode. There is no need to migrate the powered off VMs.