Take a whole vazhaipoo or banana flower. Peel the outer skin of petals one by one. You will find small bunches of florals inside the petals. Remove them one by one. Make sure you put them immediately in the vinegar water to prevent oxidation. You will fine a tall stiff stigma and transparent petal like. You have to remove the calyx and stigma and discard that. You need the banana floral alone. Discard the stigma and calyx.
Soak chana dal for 2 to 3 hours. Strain it completely and set aside.
Take cleaned banana flower in a blender. Grind coarse without adding water. Add it to a bowl. Take soaked chana dal and grind them coarse without adding any water. Take it in the same bowl.
Grind the ingredients given for grinding like dry chilli, garlic, fennel seeds, curry leaves and coriander leaves to a coarse mix. Don't add any water when grinding. Add it to the ground chana dal and banana flower mix.
Add in onions, green chillies, asafoetida and salt. Mix well. This is your banana flower vadai mixture.
Take small portion from it and flatten it slightly. Drop in hot oil and fry for 1 to 2 mins. Drain and serve.