I don't see how Vettel has been immature in any way...
I only see it as him being naturally more enthusiastic in his post race celebrations as compared to other drivers, that's all. (Which feels nice to see for a change)
Apart from that, I don't see how he has been immature, especially with how he has shown much improved maturity in terms of racecraft, and extracting the maximum potential out of his cars in qualifying in particular, and the race.
After taking all this into account, I feel he's quite mature, especially for a driver of such a young age really.