During business hours, Vanguard should be able to quickly sell any of your treasury bond holdings including TIPS. If you need the money, by all means go online to the Vanguard bond desk, read the online help, read the Vanguard fee schedule, then sell whatever you need to sell.
If your situation has changed, so that you now believe your long-term asset allocation should be different than you current asset allocation, or your "reason" for holding TIPS has changed, definitely go to the Vanguard bond desk and sell whatever you need to sell.
However, if your "need to sell" is just based on the idea that by selling you can make a little more money than by holding your TIPS to maturity, then follow haha's advice.
OP, do yourself a big favor and just stand pat.
You are almost certainly not going to beat the market by churning your own portfolio's holdings. For the vast majority of investors, the more they trade, the more they hurt their portfolio's performance. The market is not perfectly efficient, but it is efficient enough that you are not going to consistently outperform it on a risk adjusted basis after taxes, spreads, and commissions based on publicly available information.
