One thing I'll say is that on first meeting, Astarion isn't so clearly evil. He's an ass, but he also just escaped from mindflayers, who clearly had people mind-controlled, as could be seen on their ship. I agree that we should be able to attack him on first meeting, but I don't think that as a Vengeance paladin we'd be obligated to kill him. Also thetenants of the Oath of Vengence explicitly say that "ordinary foes might win my mercy," and clearly differentiates between greater and lesser evils. At this point, the worst we can say about Astarion is that he's maybe a liar and definitely a suspicious asshole. And I'm speaking as someone who litereally just ran into that scene an hour or two ago, so it's very clear in my mind.
"When evil forces slaughter helpless villagers, when an entire people turns against the will of the gods. when a thieves' guild grows toa violent and powerful, when a dragon rampages through the countryside-at times like these, paladins arise and swear an Oath of Vengeance to set right that which has gone wrong."
That all sounds like a cut above "an asshole who attacked me at knifepoint." So I don't think we're in danger of becoming oathbreakers because we don't kill him.
