A VLAN will isolate macs so that only those adaptors in that VLAN can see each other. Granted, there isn't really a concept of a netmask based subnet, but then that's because you don't really have control over one's physical address.
Now, you can have an adaptor in more than one VLAN, which is the point of them. As I said its not a perfect analogy, but then they are there to achieve different things based on different semantics.
A subnet should only be in one vlan, but there are networks where there is more than one subnet in a vlan.
Whether that is appropriate or not, that would be a different topic.