To give a direct answer, mutual fund values ("NAV", for "net asset value") are updated once a day, and that new value is shown in your Vanguard account every day.
This is slightly different than individual stocks or ETFs, where the value is updated constantly (like, every second) during open market trading hours. I don't own any ETFs, so I'm not sure how frequently Vanguard updates their value in your account, but I'm sure it's also once a day at a maximum.
So you can check every day if you want. Or just look at a 3rd-party's finance website (like the Google Finance link dividendman provided) to track the value over time.
But don't do that, because you're invested for the long-term, and it's a waste of time and mental energy. The change in value on any particular day is meaningless.