Depends on the breed, but my ducks don't really fly or jump. They might flap their wings to help them get over a small slope, but none of them from the small Welsh Harlequin to the chunky Pekin is able or willing to jump a meter high.
Female muscovies are pretty different from mallard derived ducks. If you clip their wing feathers they should be fine, but otherwise they definitely can jump that fence easy.
It might be a bit claustrophobic. They are social animals and prefer the company of a few other ducks. Too many in that space might be fairly compact, but if it is just for slug hunting duty they should be okay with a larger area to roam.
Any reason you can't fence the veggie patch in and let them free-range the entire area outside it?
It really depends. If you stuck food at one end, and they saw you do it, they'd go in. But, ducks like to stretch their wings.
I had to convert the cm to inches, as my brain isn't as versed in cm. (for others like myself, it's almost 4 feet wide and just over 3.5 feet tall). The wingspan of a muscovy is 4.5-5 feet, so they wouldn't be able to stretch their wings. So, they probably wouldn't spend too much time in there.
BUT, I've found that with my mallard ducks, that after a year of having them on my property, they've destroyed the slug population so thoroughly that I don't see slugs, even far away from where they usually roam.
How big is your vegetable patch? It may be that just by having the ducks near it, they will eat so many slugs that wander over to them, that they will pretty much destroy the slug population, without having to spend the time and expense of making a duck tunnel.
It's a 12m x 45m (40ft x 150ft) salad crop, small commercial scale. Slugs can make a big difference to the income from it. The 'triangles' will cost a few hundred pounds, but that's only a couple of weeks income from the salad.
I've dealt with a lot of slug destruction, myself, before I got ducks. Seeds didn't even spout out of the soil before destroyed by slugs. THen I got ducks. I usually have about 8 of them, and they free range but usually stay by the house, but I still don't see slugs much 100+ feet from where the ducks are. I'm thinking you could do the duck tunnel, and if you fed them at the other end of the garden bed, inside the tunnel, and gave them a pail to drink from down there, they would waddle down happily to get the food, and eat slugs as they go back. After a year or two, you probably won't need it, as they'll have destroyed the slug population. (I only see my ducks eating a slug maybe once every three months, and it used to be that there were slugs EVERYWHERE).
How many ducks do you think you'll be getting? I'm thinking 5 would probably do the job very well for you. Even just three might do the trick