It's very strange that this game needs 7/8: its system requirements are very low (1.7ghz dual core, 1GB ram, 256MB VRAM). The game is just coded to refuse to run on anything but 7/8; Vista and 7 are so similar that I've never ever known of a case where anything can run on one but not the other.
It's really disappointing in any case, and inexplicable that the devs haven't commented or explained it at all.
(Also someone on the Steam forum claims to have run an earlier build fine in 32-bit Vista
, confirming that it's just coded not to work on pre-7 OSes for some reason.)