In trading on Thursday, shares of the Vanguard Mortgage-Backed Securities ETF ( VMBS) entered into oversold territory, changing hands as low as $51.0501 per share. We define oversold territory using the Relative Strength Index, or RSI, which is a technical analysis indicator used to measure momentum on a scale of zero to 100. A stock is considered to be oversold if the RSI reading falls below 30.In the case of Vanguard Mortgage-Backed Securities, the RSI reading has hit 28.3 — by comparison, the RSI reading for the S&P 500 is currently 40.3. A bullish investor could look at VMBS's 28.3 reading as a sign that the recent heavy selling is in the process of exhausting itself, and begin to look for entry point opportunities on the buy side. Looking at a chart of one year performance (below), VMBS's low point in its 52 week range is $51.0501 per share, with $53.00 as the 52 week high point — that compares with a last trade of $51.09. Vanguard Mortgage-Backed Securities shares are currently trading down about 0.5% on the day.