ETF winners and losers for March 15, 2012: ETF Winners: iPath Dow Jones-UBS Subindex Natural Gas ETN: (NYSEARCA:GAZ) +6.7% MarketVectors Viet Nam ETF (NYSEARCA:VNM) +5.9% iShares Dow Jones Transportation (NYSEARCA:IYT) +3.3% ETF Losers: United States G...