By vandals, do you mean graffitti, wanton destruction (smashing shed's etc) or theives (pulling up food)?
Getting kids involved with community stuff is hard, mainly because they have to want to own the thing...as opposed to being given something and then told they are responsible for it, which is what often happens.
Not all kids have an attitude of gratitude when they realise how much work the thing turns out to be. They are very socially orientated and commitments can be very adverse, if the whole crew aren't in on it.
It may be worth recruiting a tag team of graf artists, providing them with a theme and letting them have the walls, if there are any... That may work, if the team has decent cred, they'll establish the patch, so to speak... There may be some cross tagging, but at least you know your team will keep it looking good, and rework any damage. You'll probably need an older artist to help pull that together. Try your local tattooist, if he doesn't airbrush, he'll know someone who does.
If it's wanton damage... A free sausage sizzle / hotdog stand on a Friday or Saturday night could insert a debt. Kids are into accounting. You give them something - a feed, a laugh, acceptance - and they will stand up to defend their right to it.
Theives, well... As all descendant of convicts know

, if you can eliminate the habitual and compulsive, then you are just left with need. Meet the need.
Change will come from the inside - not the inside of the kids - as the older generations often insist, but the inside of the group. Todays Adolescents and Young Adults are very social creatures, very group centred. This could be why you were approached Andy? You present as youthful and non-conformist. Well, I haven't seen a recent photo, but you know what I mean? You have been seen to have established a group and led it peacefully and postively.
Groups go bad when their leadership is bad. Get the same group of kids with a strong value based leader (or leader influence) actively projecting direction, ideas or activity goals into the group, and they often come good. Amazing how often a good looking
good girl, can transform a whole group of boys... and vice versa.
Find the leader by inviting the group to come to you... Put on free food and they'll be there, you just have to find them in the crowd... earn his or her respect... share your values when she or he is sharing theirs... IF the leader considers you solid, and your idea(l)s worthy and truth based, they will pass yours on, just like you will pass theirs on. Most young people will respond well to the
ish concept I think - it leaves room to move, and its flexible. If you can't connect, then pass on the problem. Refer.
Young people respond to reality. Introduce them to "Old Tom whose had 2 Strokes and still creeps down the hill each day to weed his patch." and "Mary who is feeding 3 kids on her own out of her patch." and "Joel whose a bit shy, so he keeps himself busy in his garden." Make sure they understand that the idle patches are held by folks who just stopped caring, but the busy patches are need based projects. Kids don't want to know you care about them, they roll their eyes at that - they are learning to care for themselves and don't need any other confusing opinions on the matter - but they do want to know you care about something. Address the crowd and tell them why you care about these gardeners, and why you care about what they are doing. The kids'll be listening.
Might take time, but someone obviously thinks you can make a difference Andy. They brought this problem to you. Go for it - Jump in and see what you can do!
Hurt people hurt people, Andy. And they damage gardens, and take belongings. Whatever folks think about vandalism, those kids could still be tomorrows gardeners.