I reckon the Half life VR series will be spin offs, like opposing force or Blue shift being Side storylines to that of Gordon Freeman. I reckon Alyx and other subsequent Half Life VR games, are a test to see how people react to full length VR games as well as other technologies/game mechanics to include or further develop in further titles. Half life 3, 4, etc will be developed for both VR and Non-VR experiences,  based on what they learn from the side games such as Alyx.