Could you please let us know what version you are running. It would also be useful to see the HA group configuration if you can post it please.
In this situation for the configuration synchronisation, I'd suggest ConfigSync is the way to go as I'd imagine it's simpler to setup.
Regarding the dedicated VLAN I'd suggest most debate has been around a dedicated interface rather than a VLAN. In theory this would be best practise but in reality, if a switched network was so congested that traffic was being dropped it's not going to help much unless the two devices could be directly cabled together which wouldn't be the case here.