At least the first season is a series of plays Vince and Howard put on for their parents.This explains the episodic nature of the show, the Negative Continuity, the purposefully low budget look of the set pieces/costumes, complete lack of a fourth wall (as children often break character to explain bits to their parents), and the overall whimsical nature inherent throughout.
The entire show is Howard and Vince acid tripping together.Naboo's their shady dealer.
Vince's increased bitchiness is a result of heartacheIn season one, Vince is very placid and good humoured to most of Howard's egotism and character tics, but in season two he becomes a lot more self-centred and combatant towards Howard. This is because in season two he realized he was falling in love with Howard and wasn't dealing with it well, so he Took a Level in Jerkass as he went into denial over it. When his brain cell in "Journey to the Centre of the Punk" said he didn't feel anything for Howard, he was lying, either because his denial was that deep, or because his brain cell wants what Vince wants, which is for Howard not to know. This is supported by Vince realizing he loves Howard later on in the party episode.