I was using it for a quick check on my wife's account, which happens to consist almost entirely of Vanguard LifeStrategy Conservative Growth Fund (VSCGX).
Here is what the Portfolio Analysis tool reports for a hypothetical account that is 100% VSCGX and nothing else:

What's wrong with this picture? It's completely missing the "short-term reserves!" Here's what Vanguard's "Portfolio & Management" tab shows as the asset allocation for this fund:

These two pie charts are meaningfully different!
It's clear enough what's happening. VSCGX contains 20% Vanguard Short-Term Investment-Grade Fund (VFSTX). The official fund description for VSCGX evidently classifies VFSTX as "short-term reserves," while the Portfolio Analysis tool must be classifying it as "bonds."
Me, I don't know which way VFSTX should be classified. I do, however, feel strongly that Vanguard should classify it consistently!
(VFSTX has an average duration of 2.2 years. Vanguard gives it a 1 on its 1-5 risk scale, same as Short-Term Bond Index, and less than the 2 it gives Total Bond Market. Vanguard says VFSTX "Offers potential for more income than a money market fund. Appropriate choice for investors with short-term goals who can afford slight risk for the chance of higher returns.")