If you haven't noticed, using Viva 3G network consumes more battery than any other network in bahrain, zain and batelco to be specific. To make the story short, Viva 3G network is actually setup on HSPA+ which consumes more power than 3G.
Brief Technical Information: Quoted from Wikipedia:
High-Speed Downlink Packet Access (HSDPA) is an enhanced 3G (third generation) mobile telephony communications protocol in the High-Speed Packet Access (HSPA) family, also dubbed 3.5G, 3G+ or turbo 3G, which allows networks based on Universal Mobile Telecommunications System (UMTS) to have higher data transfer speeds and capacity.
zain and Batelco provide HSPA+, however, the phone has to switch to it when it requires high speed access, but on Viva, HSPA is the default.