Sorry I was getting a bit head of myself.
Network Failover uses the same address as the peer IP address setups for failover and config sync. They use that address to find out the status of the other unit. If an interface goes DOWN it will that message through the network to the other peer signiling a failover. If the secondary lost connectivity to the primary, it will attempt to perform a takeover. This is assuming the primary went off the wire or powered off. If not and the only thing you lost was looking at each others from the peering perspective then you have a situation where both are going to active/active - which is not what you want.
VLAN Failsafe does rely on the peer for one item, which is the overall cross communication of each other's status. SO if VLAN Failsafe or Gateway failsafe detects a problem it's going to want to send infornation to other unit on its status.