Blackdog, it has nothing to do with the fund prospectus. Vanguard, and many other fund managers, want/need to protect their investors from 'false' blips. By allowing people to pull in and out of funds at any time, false negative blips in the price will happen. Thus many, I want to say most but have done no real statistical analys, penalize their investors (fees) if they pull out of a fund with in a certain time frame (usually 90 to 180 days). Vanguard has taken it a little further, or different, in that they won't even let you sell your shares in their funds if you have all ready performed two round trips.
A couple examples of round trips:
You buy fund A
Transfer money from fund A to fund B
Tranfer money from B to anything (including selling)
or
You buy fund A
Sell
Buy fund A again
They discourage quick trades different than others, they basically don't let you trade your funds at all if you have done 2 round trips. Others simply say, you can't sell for 90 days without a penalty, but you can sell/trade as often as you want if you hold for more than 90 days without penalty.
It's all about protecting their investors. You have to understand this. And it's not part of the fund prospectus, it's the companies practice. Obviously Vanguard is a little more suited to what you want to do than most others, but there is still that rule.