In a shallow, resealable plastic container (big enough that most of the cucumbers will be covered with the liquid), whisk together the vinegar, water, agave, and salt until well combined. Stir in the cucumbers (they should be at least halfway covered in the liquid). Cover and refrigerate for at least 6 hours or overnight, stirring at least once. Divide the cucumbers among 2 serving bowls and top with sesame seeds, if desired. Serve immediately.