All failover related communication will take place over whichever HA/heartbeat VLAN you have specified. If configured, the mgmt interface of each unit can also be used as a backup. The serial cable can be used as an alternative.
From a design point of view, if the units are close enough together I'd use the serial cable (max 50ft I think). If you can't do that, can you directly wire a dedicated Ethernet cable between devices for the HA VLAN (100m). If you can't do that, make sure your management interfaces are connected using different infrastructure to your HA VLAN related interfaces.
Oh, and test it to death!
Cumulonimbus
