Why not 1.4g? Because that's high enough that human characters would be noticeably affected by it and would be likely to remark on it, neither of which is the case in canon. What we see onscreen is that Vulcan gravity is not noticeably an issue for humans; therefore the logical conclusion is that it's fairly close to Earth gravity. Even Mandel's 1.251g might be a bit steep.
OK, now this is fair.
Note on notation: According to http://en.wikipedia.org/wiki/Standard_gravity
, one standard gravity is often written as g,
although they claim that usage is incorrect. http://en.wikipedia.org/wiki/Earth's_gravity
clarifies the notation as correct, though (allowing two different senses of it).
The Memory Beta page at http://memory-beta.wikia.com/wiki/Vulcan_(planet)
depicts Vulcan gravity as "1.4 G", with a capital G. Looks like that is nonstandard.