The 28 levels in question comprise the Invasion challenges, a hardcore speed-run mode that mashes existing levels together. Posters on NeoGAF note that the total number of collectible Teensies in the Vita version is reduced from 700 to 615 as a result.
When the game was announced for Vita, Ubisoft stated that it "includes all of the original Rayman Legends content, as well as five exclusive maps". It's out now in the US and Australia on Vita, but has been delayed to September 12th in Europe.
http://www.ign.com/a...ssing-28-levels
V____V Vita..