That is one of my personal bones with Vanguard, too. I have not found a way to do it either.
The closest thing for me is to go to the My Portfolio tab, Performance, then at the bottom of the page there is a link for"View current value versus net investment graph". Since I have not put anything in this pot for years, I just eyeball the graph at points of interest, jot down the estimated values and calculate the return by hand.
It should be possible to do a custom ROI, even a Discounted Cash Flow that takes into account new money using a lilttle Java applet, but no. This site is as bad as trying to track investments with Quicken, maybe worse.
Still, under Fidelity's web site for my 401k, "Performance" simply shows the performance for the funds available in our plan, so it is even worse.
What galls me is that Vanguard sends me flyers (printed with MY MONEY) that pat themselves on the back about how great their web site is. Pththhthth.