What if Elena couldn't be compelled? Because she is a doppelganger she can't be influenced by vampires. It didn't use to be that way. For example when Katherine was human, but over time the universe saw the need to make the human Patrovas immune to the compulsion of vampires. Elena was the first to get the immunity. She didn't know this and neither did anyone really. The vervain wasn't what made her immune to the compulsion. It was her own genes. Stefan hadn't ever compelled her, but Damon had. The first night they met. She remembered it, but she didn't know how important it was until she saw him again.
She thought that maybe her father had been giving her family vervain while he was alive. He did know about the council after all. She didn't really figure out that wasn't true until the next time she was compelled, without her necklace. It was her encounter with Elijah… he pulled her necklace off and tried to compel her. That was when she knew something was off. So the night that Damon told her he loved her, but compelled her to forget… that was the night she started to fall for him. The only thing was, she couldn't tell him she knew. She had to let it all play out because she could feel it deep in her bones to keep it quiet that she couldn't be compelled.