I use MX Player (free on the app store) to play my videos and one of the cool things about it is if you simply slide your finger up/down along the left side of the video it controls brightness. If you slide up/down along the right side it controls volume. Moboplayer also does this but I prefer MX Player. I usually watch all my vids at full brightness but use the phone at a set 50% brightness.
If I remember correctly the setting for video brightness in the stock player is available in portrait mode but it's hidden in widescreen mode. There are too many options to fit vertically in widescreen mode so they hide some.