Toward the philosophical side of your question, it seems to me that what is most important is knowing how well your fund is performing versus it's benchmark. This is an actionable piece of information that can help you get out of an under-performing fund, although if you're already using Vanguard it's likely a low cost and broadly diversified fund. Ultimately, what you want to avoid over the long term is under-performing the market due to high fees, market timing, poor fund selection etc., and selecting a fund that closely tracks the market seems to be the best way to achieve this, assuming that you intend to be a passive investor.
I don't see a clear benefit to calculating a personal rate of return. If the fund is performing well versus its benchmark, you are likely to stay with it, and if it is performing poorly, you are likely to pull out. At the end of the day, the complicated accounting won't actually change the amount you've got in your account, so I'd recommend picking a good fund, checking up on it once in a great while, and putting your time to better purpose.