Rottweiler 22, I was always under the impression that the payment holiday of 18 months was effective from the date of taking out the loan. So in fact you'd need to be pretty much going straight from training into a highly paid job.
Could be wrong (it's been a few years since I was researching that side of things) but that's the impression I got.
I was always under the impression that the 18 months start when you finish your training, but I could certainly be wrong. Then again, it makes complete sense that the 18 months starts when you take the loan, which would certainly be a game changer for some, like you said, having to go straight from training into a job. I can't find any info on it from the BBVA, it just says that years 1-2 are a "repayment holiday", no mention when this holiday starts immediately or after training.