ARP proxy can be used when RTU’s IP addresses behind different RipEX units are for any reason within the same IP subnet, typically they do not have routing capabilities.
VLAN feature is typically used when you need to split the network into several logical parts. E.g. to distinguish between management and payload (user data) traffic or among different applications traffic (e.g. various RTU technologies).
Both features can be combined to provide the necessary functionality.
See the following chapters for a detailed description.