EMERGING MARKETS-Vale, OGX help Brazil's stock index rebound from sell-off

* Brazil's Bovespa rises for first session in five * MPX Energia jumps after Batista steps down as chairman * Bovespa rises 2.1 pct, Mexico IPC up 0.77 pct By Asher Levine SAO PAULO, July 4 (Reuters) - Brazilian stocks rose for the first session in five on Thursday as bargain-hunters picked up shares of commodities producers OGX and Vale to take advantage of recent losses. Mexico's IPC index rose slightly, while Chile's bourse capped a three-day decline. Traders said a m