Ask yourself a very simple question - would the authorities in the UK have been any different if you had arrived without the correct paperwork? I very much doubt it.
The vignette in Croatia is required for all foreign flagged vessels and should be purchased as soon as you clear into the country. If the authority in Dubrovnik couldn't clear you in straight away then you should have gone to the nearest harbourmaster/kapitanja to obtain the paperwork. If you arrived on the 17th did you not try again on the 18th, 19th etc etc? As others have already said the vignette is the one thing all the officials look for so it simply is not worth risking sailing in Croatia without it. But then having been in the country for 3 years you should know that.
