That's pretty much been confirmed by TENAA having just certified the X6S for sale in China. As always, this has resulted in both images of the device becoming public, as well as the list of its most important specs.
The vivo X6S has an extremely thin metal build as you can see, a fingerprint scanner on its back, a 5.2-inch 1080p AMOLED touchscreen, a 1.8 GHz octa-core processor, 4GB of RAM, 64GB of storage, a 13 MP main camera, an 8 MP selfie snapper, and 4G LTE. It runs Android 5.1 Lollipop. Its dimensions are 147.9 x 73.75 x 6.56 mm, and it weighs 140.5g.
With MWC now fast approaching, perhaps vivo is waiting for that event to finally announce the X6S and the X6SPlus. Even if that doesn't happen, the two handsets should be unveiled in the coming weeks.