If damon can sense vampires, why didnt he know he was making out with Katherine?

I thought exactly the same thing, but then I remembered something from the books. If you read them you know they're different from the show, but when Katherine showed up on the books it was because she wanted them to know she was there, but before that they couldn't sense her. It happened because she's older and stronger than both of them, so even though they (especially Damon, since he's stronger)looked for something that might have been causing some bad things that were going on the town, they couldn't find her. She was hiding her presence from them, so I THINK that's probably what she did on the show too.
I hope it helped! =)

i think it was because he was distracted, he's crazy about elena, but if you think about it,in the books Katherine was a cat in tye house for a while and damon of stefan didnt know, but wat i thik is that he knew something was up..... cuz why would elena kiss him?... anyways so yeah thas my theory. hope it helps.

Vampires can't sense other vampires i think. Stefan said that to Elena in Unpleasantville.

i think he sorta did coz when she went inside he looked like he was familiar to the taste

I think Damon was weak from the vervain and he was distracted also.

if damon can sence other vampires and he did not know it was katherine it could be because katherine was "masking her powers" like she did in the books.(in the books she was the one that lead stefan and damon to fell's church(mistic falls) and they never knew that she was doing it)

Vampires can't sense other vampires, but they can 'sense' humans especially when they kiss (like Stefan,Anna etc). But Damon was kinda of distracted ,vulnerable and surprised that Elena wanted to saved him, about the fact that he kinda feels human again etc, so he didn't notice that she wasn't Elena right away. I think he did realized that after they kissed when he touched his lips..