The machine does not define if it is multi domain or not. For a Multi-Domain management you need a separate machine anyway, it cannot be run on a gateway that works as a VSX gateway.
A VSX gateway can handle as many domains as the license allows you to run VS's on it. So if you have a Multi Domain server and a cluster of 15600 in VSX mode, the only limit is your licenses, the one for the number of VS'es the other for your Multi Domain server.So when you have 2 15600's with each a 25 VS license on them you can tun 25 VS'es shared across them or all on one box or when you run them not as a cluster 25 each. Then your Multi Domain server license needs to support 25 or 50 domains if you want to run a VS per domain.
We have all kinds of customer setups where we have customers with a single gateway with 3VS'es on top (single domain), customers with a cluster with a single VS on top (single domain), a customer with 2 clusters with 15 VS'es each (single domain) and shared environments with a cluster gateways running around 25 different customers (1 domain per customer).
So it all comes down to what YOU want and need.