Clean vazhaipoo and chop it fine. Soak chana dal in water till immersing level for 1 hour at least. While you do other works, keep chopped vazhaipoo immersed in water + a tablespoon of buttermilk just to avoid browning.
Drain water, Then add chana dal and red chillies in a mixer and grind it coarsely. Drain water completely, just squeeze and remove excess water from vazhaipoo and just give a quick grind for vazhaipoo (just one or two pulse) and add it together in a mixing bowl.
In a mixing bowl : add vazhaipoo, chana dal, onion, curry leaves, fennel seeds powder, rice flour and required salt. Mix well to form a dough consistency. Shape them into medium sized patties/vadas.
While you shape the vadas, heat the oil. Put a small pinch of dough into the oil if it raises immediately then the oil temperature is just perfect.
Drop the vadais (may be 2-3 at a time) in oil and flip both the sides and cook till nicely browned and its crisp. Fry the vadais in medium flame. Drain in tissue paper.
Serve Vazhaipoo Vadai hot with coffee / tea!