The key to Rogue or Thief as a swashbuckler is in feat and background selection.
The Noble Bred For War background gets you +1 initiative and training in any martial weapon (choose Rapier, obviously).
The feat Flash of the Blade says that while you wield a rapier, you can still deal sneak attack damage to your target even without CA, as long as you are the only one adjacent to it. Presto! 1v1 dueling with buckets of damage. Bonus points for using either Sly Flourish or Feinting Trick (it does have a use and this is it). Sadly, those two powers are not stackable.
If you have half decent strength, you could use Riposte Strike.
From there, you either go the Two-weapon Fighting and Two-weapon Defence route for slightly more damage, or you go Light Shield proficiency with Blade & Buckler Duelist for better defences. If you can grab a Rhythm Blade, you can have the best of both with the TWF build. Using a Parrying Dagger can further help your AC, and though you need to spend a feat on it, a Rhythm Parrying Dagger nets you +3AC/+2Ref with all those feats.
I think an Ironsoul Monk would be a close runner-up to this if you reflavoured it. Flurry of Blows gives you some multitargeting ability and minion control that the Thief lacks. Careful power selection makes it less an ssue for Rogues.