Regardless of Vacuum, you need to power your Element Zero core and lower the normandy's mass. Without it, there's no faster than light speed and your stuck spending the next few hundred years reaching your destination.
Presumably, you're capable of getting back on emergency reserves when you run out. Don't know why, just is.
There's quite a few oddities like that. For example, apparently you can't make faster than light jumps INTO an object. That one's apparently another plothole:
On ME2 galactic news, it was revealed that terrorists had used a shuttle to FTL into the Palavan capital. Destroyed an entire city with incredible force. However, when the question is posed in ME3 if you can't rig entire dreadnaughts to using FTL into the reapers as a desperate measure, it is answered that FTL is based on Reaper Tech and that you just can't.
#8524866, By Skirlasvoud Mass Effect 3 *SPOILERS THREAD*
Skirlasvoud 3,696 posts
Seen 36 minutes ago
Registered 8 years ago