The pins on the layout could have global names, and then it should work. However, the best solution would be to use inherited connections. If you create pins on your schematic for vdd and vss using a "netExpr" (defaulting to (say) vdd! and vss!), controlled by a properties vdd and vss respectively, and also use Layout XL to generate the layout - you'll have similar pins on the layout, and also on any resulting extracted view.
There's then no need to have the pins on the symbol - you can keep that having just the input and output pins. You'll also then have the ability to reuse the inverter in various supply domains by adding netSet properties at a higher level to override the values of the supplies.
This is well covered in the documentation - probably search for "inherited connections" or "netExpr" or something like that.