Long story short there was a recent change in regulations. The SEC is trying to take something very simple, money markets, and make them very complicated because a very small minority of money markets had problems back in 2008.
Instead of confusing you with all of those regulations and having to subject you to a lot of new complicated rules Vanguard is just switching you to a money market that is exempt from the rule.
The new rules require that most money market funds fluctuate like bond funds instead of being fixed liked a savings account that earns interest. Certain money markets are exempt so Vanguard is switching a lot of accounts to the new one.
