This article is one example of why I think Vanguard is an honest company. I'm probably preaching to the choir though.
To summarize, Vanguard has noticed large inflows to their energy mutual fund. They caution investors to take a diversified, long term approach to investing. They are doing the opposite of playing up the performance of their mutual fund, and in fact warn that it may not make sense to pile into this hot sector fund.
They also list many of their other index or broad market funds and the percent of each fund that is invested in energy. Most of these broad market funds have 8-10% in energy assets already. You may already be invested in energy and not really be fully aware of it.