2 ways that I can see immediately:
If you added VLANs to monitoring during your initial discovery for NPM, you can go to 'Manage Nodes' and simply search for the VLAN # as an interface and you will get a result set of all instances you are currently monitoring.
You could also do this as a custom report using sql as so:
SELECT n.Caption, i.FullName FROM Nodes n JOIN Interfaces i ON n.NodeID = i.NodeID WHERE i.FullName LIKE '%VLAN%'
Just change the VLAN to whatever you're searching for (VLAN #)
The second way would use NCM. I would probably recommend this because you're limited in NPM to what you've actively decided to monitor. Using NCM, you are looking at actual device configurations.
In NCM, you would make a Policy Report to search all configurations for the string (you can use RegEx as well).
Essentially, you make a rule that says "look for this"; you assign that rule to a policy and then select the devices you want to search through; then apply that policy to a report.
It's explained fairly well in the NCM admin guide under Managing Policy Reports.
Good luck!