There's something I don't understand.Why when Katherine appeared in the last episode pretending to be Elena,nobody(Mikael,Klaus)found that she was a vampire?Couldn't they smell her....?

I guess they were so preoccupied with trying to make the plan go through, they weren't thinking about one of the simplest parts.

That's the usual question... None of the vampires ever gets the difference... Theorically they should be able to hear the heart beat, smell a living or a dead body etc, but I guess for plot they need the Elena/Kat exchange or other situations, so they pass over it...

Well they see have vampire blood pumping around their body because they do bleed, so that could explain the reason why they can't recognise the difference in the heart beat? However the smell that could be a possible difference? But excellent question!