Honestly, I think the nightlife of Las Vegas, coupled with the workers of Vegas would not allow many folks from Vegas to attend the games.
Plus, is anyone really a native of Las Vegas? You might have a situation that might make Tampa Bay look like a vital baseball town
Portland is the viable option, IMHO. Move the Pirates to the East (with Philadelphia) The NL West would be: LA, SD, COL, SF, ARZ, & PORT
The other option involves switching leagues and isn't viable, nor would it be approved.