I have used Zylkene for a few different dogs. For my highly strung, very anxious rescue Lurcher it was useful for giving before anything that might worry him and did seem to help (eg kennels, long car journeys etc). We only used it alongside a carefully managed behavioural programme though.
Another rescue had it daily to take the edge of his generalised anxiety and it seemed to help, but we used it alongside Dorwest Scullcap and Valerian eventually and that was better. He was always a worried little thing, but just coped with life a little better with the help of the above.
For our genuinely fear aggressive rescue it didn’t even touch the sides. He was put on Prozac by the vet and we worked through an intensive behavioural programme with a veterinary behaviourist alongside that to reach a point where he could cope with life and ignore things he was worried about.
There’s no harm in trying it, but as others have said, these things don’t really help on their own, if they do - and it seems to be a very individual thing, they work by bringing down arousal levels to the point you can do the behavioural work needed for them to learn effectively.
As an aside, if my vet was rude and rolled his eyes at something I was saying I would be looking for another vet. You need to have confidence in your vet and not feel too worried you will be judged to be able to explain the problem and ask for help when you need it. I’ve only every had something similar with a vet once in 35 years and although we stayed at the practice, we have never booked another appointment with that particular vet. We did have a vet for years that was very direct and not at all touchy feely, but they were also extremely professional and would never have been deliberately rude or judgy. They were lovely with the animals they saw, they just kept things very formal and professional with the owners. Lots of other people refused to see them, but they were actually our favourite vet, because it meant consultations were always professional and effective.