Budgies are exceptionally stubborn.
So I would buy a nice bit of leafy green stuff- spinach, romaine lettuce, kale.. clean so it's dripping wet and lovely and pop it on the floor of the cage when it's been freshly cleaned.
They might not take to it straight away but they can't take to it at all if it's not there.
Keep trying with vet rather than fruit (it's better for them) and just keep trying. Present it in different ways. I used to chop veg up for them but mine like a nice few spinach leaves scattered on the cage floor. When I drop it in, morning or evening feed times. I sit down and I can hear the 'plop' as the budgies drop to the floor of the cage to get to it.
You might also want to limit the amount of seed they are having access to when you try and feed veggies so they don't stuff themselves full.