Practising criminal law is a vocation these days because the money is so poor (unless you get into white collar crime). If you have a passion for the job, do it! Please note, however, that the study of criminal law and the practice of criminal law are different. Criminal law is very interesting as a theoretical subject. In real life, criminal law is gritty, painful, and can be depressing. I salute those who can handle it. It's rough stuff.
I add that many or most people aspiring to legal careers do not figure out what kind of lawyer they wish be until they have studied the law for a while. If you had told me some decades ago that I would be a Commercial Chancery barrister I would have laughed in your face. As the old airline saying goes "last week I cudent evn spel the word Pielet, but now I are wun".
If you can study criminology whilst also covering the core subjects so that you don't need to do a PGDL, fine. Otherwise, study whatever subject you like and then do a PGDL, or do a law degree which includes the core subjects. Some law degrees include a criminology option.