Vance laughing at his depiction in the show was the best of all possible reactions and it bodes well for his future political career.
People gotta remember that JD is only 41. He was 13 when the first episode of South Park aired. If I was in an episode of something I grew up watching, I wouldn't care how much they make fun of me, it'd still be pretty cool.
Trump is naturally hilarious, but professional comedians making fun of him usually fail hard.
The reason is this - Trump makes them mad, you can't be funny and seething with rage at the same time. It's why Colbert is reduced to screaming kindergarten insults about the stupid poo poo pee pee Mango Man Hitler to pity laughs from the studio audience. And then wondering why that's not worth $12m a year.
I agree, but I'd like to elaborate on why this anger makes them fail. Good parodies rely on exaggerating the most ridiculous yet true aspects of what you parody. With Trump, that means poking fun at his mannerisms, speech patterns, etc. That's what Zach Hadel does, and his Trump jokes are consistently great as a result.
The problem is that a lot of people are so blinded by rage towards Trump that they don't see Donald Trump, they just see pure evil. As a result, they just make "parodies" that are completely meaningless, because you could substitute Trump out for anybody else and the joke would be effectively unchanged.
South Park's portrayal of Trump fails for this exact reason. It's just recycling the same joke they did for Saddam (which itself was a poor parody for the same reasons, and is emblematic of South Park always being libshittery that pushes the current cultural consensus).
(Edit: Just wanted to add that favorable parodies of something can fail for the opposite reason. This happens occasionally with fringe right-wing stuff that doesn't know how to poke fun at Trump. They're just making shitty propaganda that could be used interchangeably for any Republican leader.)