Think of a vLAN as a virtual hub - all computers assigned to that vLAN are essentially on the same physical segment, as far as the machines are concerned. So as long as you provide differing address spaces to each of those virtual segments (via DHCP, either directly or via DHCP relay), then the clients, once connected/assigned to the vLAN, will request for and be assigned addresses specific to that vLAN.
It's an oversimplification of vLANs, but it serves my purposes for most thinking about vLANs. :)
-Chris
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Chris Edson
SDET II, Network Access Protection Platform Team