Hi grizly32, I would encourage some form of Non-VR support is incorporated to reach the largest market. Could probably come handy development assistance and testing, and help players transition into VR to experience the game in a different way. I have to admit the Vehicle/Seating concept is innovative, a sort of Tactile Augmentation to the VR with functional real-world props. The technique may applicable to other objects. I can visualize Human Proper arrangers setting props for VR users. This may very become a form of entertainment in itself.
The Alien Invasion plot is a great pick and its one I would choose as well. It has so much more potential and allows for a decent blending of modern and sci-fi, opening the door for Humans to get their hands on some improvised Sci-fi weaponry. Fallen Skies, Battle: Los Angeles, and War of the Worlds are a few Alien Survival Horror that come to mind. Many Survival concepts (no matter theme) have common threads of overwhelming odds, a lurking terror hunting down survivors, scarcity of energy and supplies, and improvisation of weapons and tools.
I was writing a short Interactive Fiction: D Rifters in which a Family attempts to survive the invasion of inter-dimensional beings. These Beings dubbed DRifters use a energy technology that form Titanic Golem Monsters from living/non-living matter in the environment. To make matters worse, humans over exposed to the radiation from the DRifters mutate into zombie-like cannibals. The story had some other plot twists, but overall a similar formula to what you describe.