Why is mutual fund VEXPX down 7.35% today?

The fund paid out a distribution- if you own this fund you will have received $6.44 in dividend income. When this happens the stock or mutual fund that pays the dividend is said to have gone "ex-dividend". Since the fund has paid out cash to shareholders, the NAV or unit price will also be reduced by that amount. The fund actually gained $0.44 today based on it's investment results- the stated NAV change was -$6.00 but $6.44 of that was due to the dividend.

This type of event typically happens once a year for mutual funds. They typically have to distibute some of their gains for tax purposes.

Hope this answers your question.

That depends on thousands of variables influencing thousands of investors. If the fund had more sellers than buyers which it looks like for the moment, the pushes and tugs will predominantly be negative. If it had more buyers than sellers, the news would likely have been positive and you'll see an increase in price.