We spent three nights in Vernazza in the middle of March, just before the trails were pay to use, amazing 2 full days there. what a time to go!
Pro's - weather was nice (for a cold climate Canadian), just our light jackets were sufficient for everything, no real crowds to speak of, amazing scenery (without crowds), accomodation was reasonably priced for the area and plenty of choice, trains were not super busy
Cons - some places/restaurants were not open yet, some construction here and there getting ready for the busy season
Judging from the pics of crazy crowds we've seen there we loved going there at this time of year, the few cons were nothing compared to the amazing positives of being there at a less busy time, we'd do that again in a heartbeat!