For fermentation, vendai/bhindi/lady’s finger/okra or ash gourd help.

Some people have used surai/bottle gourd /Lauki.

And few people have experimented with sprouted payaru/green grams/moong also.

Ther coagulant vegetables like Vendai/ Bhindi/Okra, or  Pasalai keerai/Basella  can make the idlies softer during and after steaming.

For ensuring that you are putting enough vegetables, to match the grain quantity, you can add grated vegetables like grated cauliflower, very finely grated and chopped cabbage, capsicum, chow chow, summer gourds, etc.

Please see aKarma Recipes for more details.