No.
No it is not, "God" is a title not a name. If I said "OH MY PRESIDENT!" would that be taking Obama's name in vain? No.
Your Gods name is either Yahweh or if you're a trinitarian his name would also include Jesus.
Also saying "Oh christ" it is not taking his name in vain, because "christ" is a title as well, not a name.
Depends on what you were taught.
Most Christian sects teach that "god" and "christ" would be taking his name in vain ... Because there is only one true "god" and one "christ". As opposed to "lord" or "king", of which there are many.
However, if you belong to one of those sects ... You could always say "Oh, Zeus" and not be taking a god's name in vain - because you don't believe in that god. LOL