No, if the idea is something so basic as vampire x human or sasuke x naruto, then it's not stealing. If you on the other hand take someones characters from that pairing (if it isn't a fandom, cause then everyone will already have the same characters) then it counts as stealing as you have taken their creation.
If we counted using a pairing as stealing an idea, no one could really write anything because you could say "oh, it's a romance between a boy and a girl, that has been done before, you are stealing the idea!" cause a pairing is that basic. :p Technically speaking we would be stealing EVERYTHING that wasn't original, and the word stealing would then loose its meaning.