We ended up at Vana Belle. Service was great, as was the room (we were upgraded to a very nice suite). I did feel the property wasn't frankly what I'd expect of a Category 8 property. It lacked some finesse and finishing touches, that you might find at an RC or StR. It did not feel luxurious. Would I want to stay there again? Probably not. The beach was nice, pool area, food etc all adequate. The spa was poor - to the extent we didn't bother using it. It's a tiny reception desk, that leads to 4 treatment rooms - none of which were anything amazing (again, for a Cat 8 property at this price).
If anything, when I asked the cab drivers which were the nicest hotels in town, the ones that kept coming back were Four Seasons (natch) and the RC! So there was some remorse that we didn't just go with the RC after all. (I often feel fantastic service and communal areas can cover slightly for a poor hard-product).
I think our problem was that we'd just come from Japan, and had been spoiled at 2 fantastic properties - so perhaps nothing in Koh Samui was going to live up to that.
I will add - our butler at VB was absolutely fantastic. He couldn't have been any more helpful or responsive to every need! As was the very friendly Dutch F&B Manager, who we enjoyed bumping into several times. He makes a mean Aperol Spritz!