Hey! I had quite a few Vivienne Westwood bags back in the day, about 7 or 8 (bought between 2006 and 2010 probably). I don't know anything about this particular bag but I remember like yours some of mine didn't have any kind of branding inside which was strange, I think these were the earlier ones I bought and then the newer ones tended to have metal branded plaques inside. I always bought from the VW store or a reputable boutique. There always seemed to be a lot of inconsistencies with their bags I remember and a lot of variation.
Anyway I sold all of mine a couple of years ago and had no problems. Sold for various prices as they were in various condition, a lot of them well used. Sold them all on eBay but I sell a lot of designer items on there and have good feedback so I don't think people questioned the authenticity. Sorry I couldn't be of more help!