For reference, last year I think VTSAX paid about $200 in dividends per $10K invested. So you would owe tax on that dividend. This is generally taxed at a lower rate than your regular income.
As far as capital gains goes, if you had bought $10K worth of VTSAX in 2010 and sold those shares at the end of 2015, you would also have owed taxes on ~$6K of capital gains. These would be long-term capital gains, which are taxed at a lower rate than your regular income. If you only held the shares for 364 days they would be short-term capital gains and you would be taxed at (generally speaking) your normal rate.