Vazhai poo is nothing but Banana blossom. It takes a while to clean the blossom and prepare it for chopping. Cleaning the blossom means discarding the stamen and sheath that is attached with it. We have to do this as the stamen and sheath tastes bitter. I know that the prep stage of the blossom needs more detailing. Will surely update with more pictures soon.
Once the preparation stage is over, chop them into small bits and put it immediately in butter-milk. If not the blossom will turn black. Cook the blossom with required salt and turmeric. set aside.
Note: I just came across some good pictures about the prep stage of Banana blossom in Revathi's en_ ulagam. Here is the link to it.
Banana blossom -1
butter milk- 1 1/2 cups
moong dal- 1/2 cup(cooked)
onion- 1 (chopped)
green chillies- 4
coconut- 1/4 cup
jeera seeds- 1 tsp
Heat oil in kadai and sesaon with mustard seeds and urad dal. Then fry onions and green chillies. Now put tomato and fry till soft. Add the ground coconut mixture and salt to tomato and pour some water. Bring to a boil. Add cooked moong dal and cooked blossom. Mix everything and boil for 5 to 10mins. Finally add curry leaves.