Which langauge are you using? The reason that I ask is that the dynamic languages like perl and python can get you into trouble if you are not packaging the arrays properly. If you pass a scalar when an array is expected, the server side will interpret that as a zero sized array and thus return with no error. If you could provide more details and possibly some sample code, I may be able to help diagnose things further.
If you do want to verify a method succeeded such as the VLAN.create() call, you can always call the get_list() method after the fact and look for the entry you created.
Again, if you pass along more details we'll see what we can do to get your VLANs created.
-Joe
Cirrostratus