NeoGAF users first spotted this discrepancy and have provided solid evidence for their claims. In screenshots taken from the Vita game and its Xbox 360 counterpart, it's clear that the latter game features more "lums" for players to collect. 700, in fact. The Vita game has 615. The missing 85 lums would normally be found in the 28 Invasion levels that should be present in the Vita game.
These 28 Invasion levels ask players to complete a given stage in as little time as possible, and are seen as some of the most challenging content found in Rayman Legends.
We've reached out to Ubisoft regarding the missing Vita content.