I'd say Swashbuckler or Mastermind are your best bets for Rogue dip.
Swashbuckler gets you +CHA to Initiative, some extra mobility if needed due to the free Disengage, and most importantly the ability to trigger your own Sneak Attacks. This is the better option for boosting your own combat ability, especially if you're the only melee fighter in the party and you don't have another way to reliably get Advantage.
Mastermind gets you the ability to Help as a Bonus Action from 30 feet away. Especially great if you have another party member who's also a Rogue, or who has Elven Accuracy, but handy for just about any party anyway. Plus most Paladin's don't have many other reliable uses for their Bonus Actions, so you could Help nearly every round. This is the better option for boosting your party's combat ability as a whole, especially if you have another way to get Advantage or you have another melee fighter in the party who can team up on enemies with you.
(Arcane Trickster is a distant third choice, and gets you some cantrips, a few additional 1st level spells, and an extra level equivalent of spell slots for Smiting or upcasting. But your INT will likely be low-to-middling, so you'll basically be relegated to utility spells.)
As for Paladin Oath, I agree that Vengeance is a good choice (especially if you choose something other than Swashbuckler and need a way to generate Advantage). Primarily because Oath of Emnity lets you use your Channel Divinity to get Advantage against one enemy for guaranteed Sneak Attacks against them for the remainder of that combat, and later Haste as an Oath spell lets you Sneak Attack on both your turn as well as on an enemy's turn with a readied action for double the fun. This is the better option for boosting your own combat ability.
Another good option is Oath of the Ancients, which also has a way to use Channel Divinity to generate Advantage for Sneak Attack. It's not as reliable as Vengeance since it allows a saving throw, but it also grants that Advantage to all your allies, not just yourself. And you also get Ensnaring Strike as an Oath spell, which like your Channel Divinity lets you Restrain enemies for party Advantage. And its 7th level ability is significantly more useful for you and your party than Vengeance's. This is is the better option to help out your party in general.