If you look at the lower lip of a vlei, it has a distictive blue colour. Only vlei's have that colour lower lip.
Re the size of that vlei. It depends how well nature treated it to get bigger than 'normal'. The smaller species fishes does not grow as old as the larger species.
Vlei's are already agressive, when spawning they are extremely agressive and even attack barbel intruding many times it's size - and the barbel yield, or else they won't survive.
A blue kurper also have the red tips on the back fin.
A good catch, check for that blue lower lip.