I can imagine Vanden Plas really taking off as a standalone brand with a range like this in the 60s, which makes me doubt that BMC would then bother with Rover (even if there are obvious synergies that could be exploited via an acquisition).Roughly envision the ATL 1960s Vanden Plas range evolving as follows prior to BMC acquiring Rover:
Vanden Plas Sheerline 4-litre - Essentially an XJ Series 2-like variation of ADO58 aka Burma/Korea that appeared in 1965 (with coupe version possibly reviving the Atlantic name) and was powered by a 200-268 hp 4-litre 6-cylinder Twin-Cam.
Vanden Plas Pathfinder 3000 - Best described as a Vanden Plas version of the Austin 3-litre ADO61 that appeared in around 1963 and benefited from a probably developed 3-litre 6-cylinder putting out around 150-175 hp.
Vanden Plas Princess 2000 / 2500 - An earlier alternate production version of the X6-based Vanden Plas 1800 prototype, powered by a 112-115 hp 2-litre B-OHC and a 134-138 hp 2.5-litre E-Series 6-cylinder engine.
Vanden Plas Kestrel 1600 - Resembling an ADO16 aka 1100/1300-sized three-box version of the Vanden Plas 1800 prototype (many years prior to the OTL Michelotti styled Austin Apache / Victoria), it was powered by an 80-90+ hp 1.6-litre engine.
Vanden Plas Newmarket 1300 - A further downsized Mini-derived 2/4-door three-box variation of the Vanden Plas 1800 prototype (with similar styling elements the Mini-derived MG ADO34 prototype that replaces the OTL Wolseley Hornet/Riley Elf), it was powered by a 59-70+ hp 1.3-litre engine.