There was an IGN First video like halfway through July where they were showing off First Light. The IGN commentator asked Bungie something along the lines of "Are there vehicles that seat more than one person?". The Bungie rep replied that they couldn't comment on that. But he never said "no". I took it as there's likely multiple other vehicles, including multi-seated vehicles tightly under wraps for the release. Maybe I'm over-estimating, but hey, Halo certainly didn't have limited vehicle choice.