In a large bowl, prepare an ice-water bath and set aside.
Cup lemons into paper thin rounds, about 12 rounds per lemon. Remove and discard the seeds.
Bring a medium saucepan of water to a rolling boil. Remove from heat, and add the lemon rounds to the hot water, let them sit in the water until softened (about 1-2 minutes), stirring a few times. Drain, and immediately transfer the lemon rounds into the ice-water bath. Drain and set aside.
In a large skillet over high heat, bring sugar and 2 cups of water to a boil, stirring to dissolve the sugar. When the mixture is clear and bubbling, reduce heat to medium-low. Add the lemon rounds and vanilla beans. Using tongs, arrange the lemon rounds in one layer.
Simmer (do not let the mixture to boil), until rinds are translucent, about 1 hour.
Transfer lemon rounds to a baking sheet lined with parchment. Let stand until lemon rounds completely cooled down to room temperature.
Bring a medium saucepan half filled with water to a boil. Place chocolate in a medium bowl set over the saucepan of simmering water, let it melt, stirring just a few times. Remove bowl from heat, half dip the lemon rounds into the melted chocolate. Place on parchment paper and sprinkle with sea salt.
Refrigerate for one hour before serving for the chocolate to settle.