If a physical interface/VIF does not belong to any VLAN, you can use the vlan create. If VIF is already a member of a VLAN, you should use the "vlan add" command to add the interface to subsequent VLANs.
In your case, VIF is already created and VLAN exits on top of it.
In order to add another VLAN with network Identifier '50'.
Well, the interface_name is the interface you want to add to a VLAN. This could be physical interface such as e0a ,e0b etc, or it could be a virtual interface, simply called VIF. If you already have a VIF (with teamed interfaces such as e0a, e0b, etc), then you don't have to give physical interfaces name, instead you give VIF name. Basically you are building a VLAN on top of VIF. Isn't it.