vm_stat will spit out a generic overview of virtual memory usage, looking something like this:
$ vm_stat
Mach Virtual Memory Statistics: (page size of 4096 bytes)
Pages free: 5231.
Pages active: 130041.
Pages inactive: 73169.
Pages wired down: 53703.
"Translation faults": 84039105.
Pages copy-on-write: 7089068.
Pages zero filled: 32672437.
Pages reactivated: 432070.
Pageins: 62166.
Pageouts: 63545.
Object cache: 1445817 hits of 1470191 lookups (98% hit rate)
If you want a continual update of your virtual memory usage, try adding a numeric value after the vm_stat command, indicating the amount of seconds that pass before refreshing the data. For example:
vm_stat 3
