Victory lap is for leveling up.
Each race in grand prix you win a maximum of 10 points.
It takes 100 points to level up.
Each time you level up you get to few seconds to drive around and get coins and crates. In the crates is coins maximum of 100 coins in 1 crate.
Just keep racing in grand prix and you'll level up in no time. I got to level 10 and got the achievement half thru expert grand prix.