You can buy admiral shares if you have enough invested with Vanguard ($50K?) or enough invested in that fund ($10K?). In return, they reward you with a lower expense ratio. Other that the expense ratio, the funds are identical.
Vanguard will automatically convert you from investor shares to admiral shares shortly after you reach the threshold. This conversion is a tax free event, meaning you won't owe any capital gains taxes on the conversion even if the shares are held in a taxable account.