you can use string substitution. Using Ops is deprecated ( see http://www.rubydoc.info/github/yast/yast-ruby-bindings/Yast/Ops#add-class_method ).
you can use
```
vlan_device_name = "#{interface}.#{vlan_interface}"
```
only difference is when `interface` or `vlan_interface` is nil, becuase ycp then return nil string, but ruby construct "" instead of nil value in this form and if you use `+` then it raise exception as `+` is not defined for nil.