The bandwidth resource control applies only to the traffic that goes through the virtual switch. Thus, inter-vnet traffic is not subjected to this limit. If you do not have a physical backend device configured, you can ignore bandwidth resource control.
The minimum supported bandwidth limit depends on the Oracle Solaris network stack in the service domain. The bandwidth limit can be configured with any desired high value. There is no upper limit. The bandwidth limit ensures only that the bandwidth does not exceed the configured value. Thus, you can configure a bandwidth limit with a value greater than the link speed of the physical network device that is assigned to the virtual switch.