Dying vinca plant

Your plant appears to be Catharansus roseus, commonly known as Madagascar Periwinkle, also frequently confusingly referred to as vinca, which it isn't, Vinca is a separate genus. It does not tolerate frost and is therefore grown as an annual in colder parts of the world.

Couple of things - this plant does not appreciate soil which is richly fertile, so adding vermicompost won't have done it much good. It does better in the ground outdoors in partial or dappled shade, in ordinary garden soil which has not been over fertilized. Sterilised potting soil should be used if you want to grow it in a pot, but I understand, where you are, that can be difficult to source. Using garden soil in pots is not really recommended because there may be pathogens within it which, whilst fine in open ground, are not fine when confined to a pot, or at least, not fine for any plant trying to grow in the pot. As a side comment, the pot has also been overfilled - the soil within should leave half to one inch clear at the top to facilitate watering; if the soil is right to the top, water just runs off and the plant may suffer drought. The pot should also have drainage holes,and should never be left sitting in water in any outer tray or pot.

If it's still alive, it would be better to transfer it either to a pot with proper potting soil (and don't fill the pot to the top) or find a spot for it outdoors - keep it watered after planting. Further information here https://worldoffloweringplants.com/grow-care-madagascar-periwinkle-catharanthus-roseus/