Hello!
The vanguard retirement funds are awesomely priced, but I disagree a bit with their methodology. They (rightly so) invest almost everything in stocks at the outset.
What they do later is slowly increase the proportion of bonds, increasing the bond proportion the closer you get to the listed retirement year. (2055 for VFFVX and 2050 for VFIFX). So if you think you should be getting into more bonds the closer you reach retirement, the vanguard funds are one of the cheapest and easiest things to do. Jack is right though... you should just choose your retirement year and stick to the one fund that has that year.
HOWEVER, unless you plan on spending all your money once you reach the magic retirement year, I would argue that you should still be in 100% stocks into retirement. This is because you DON'T take all your money out once you reach retirement, but a small percent every year. This is a bit anti-dogma, but I've done the simulations for both US and Canadian citizens, and, historically, you will almost always be able to take out more money each year (even if there were crashes) in 100% stocks (see attachment).
You can read more about the logic behind this strategy on my website at
http://lrdatabase.com/investlogic/guide.htmlHappy savings!