Per 174 of Campaign Operations, "vehicles, ’Mechs, and other salvaged units may be repaired" after salvaging them. The issue is that there is no working definition (that I can find) for what constitutes salvageable. Is there an official rubric? Or is it as simple as following the destroyed vs truly destroyed criteria in Strategic Operations?
For my own purposes, I've always considered a salvageable unit one that can't leave the battlefield under its own power.
'Mechs - Pilot killed / unconscious when the scenario ends, at least one leg destroyed, two gyro crits, three engine hits, two hip crits
Vehicles - Crew Killed crit, fusion engine crit, cruise MP reduced to zero through motive crits, turret destroyed, crew bailed out
Aircraft - crash landed, can't take off