The questions posted in the comments help to answer this question:

 - Humans were the key founders of the Federation.
 - The Starfleet Command HQ is located on Earth.
 - Vulcans are not particularly interested in colonisation and space exploration, but humans are.

Additionally, two of the three major Vulcan characters in Starfleet (T'Pol and Spock) were both strongly discouraged from joining (T'Pol largely due to strong prejudices against humanity during the pre-Federation era). Because Starfleet started as an Earth organization and with longstanding Vulcan prejudices against humans (apparently we smell bad and are uncivilised) and Starfleet itself, Vulcans simply weren't enlisting in the same numbers as humans were. Science and diplomacy are apparently more respectable professions for a Vulcan. So even if they did enter Starfleet, they'd probably gravitate towards those specialties rather than command. Even Tuvok started off as a science officer.

Plus, TNG and DS9 implied that Vulcans and other species commanded their own fleet of Starfleet ships. It's possible that there are regional divisions in the command structure.

Out of universe, a lot of the named Admirals in the series are simply named after producers and others who worked on the show (e.g. Peter Lauritson, Rick Berman, Michael Piller, Richard James, Marvin Rush, etc.), all of whom happen to be human.