Mutual fund giant Vanguard has added more than 250,000 shares to its National Health Investors stake since the end of 2011, which takes its stake in the Murfreesboro-based real estate investment trust above 10 percent. Vanguard, which is easily the largest institutional holder of NHI stock, disclosed its holdings in an SEC filing Tuesday. NHI shares (Ticker: NHI) closed Tuesday trading at $49.46 and are up 12 percent so far this year.