A vignette is a small scene sitting on a base (ideally not much larger than 8 by 8 studs, at the very most 14 by 14 studs), focusing on a single event or maybe even a more abstract theme. For these vigs, we're asking you to come up with something that could fit either of the two titles above. "Army Life" lends itself to a vig depicting common scenes from basic training, soldiers relaxing between missions, etc. "Rescue" is fairly self-explanatory and works well for any number of action-oriented scenes. Both of these subcategories can be interpreted in any time era you like, from late Imperial up to a futuristic setting.
This was recently discussed on LAML radio.