Idli vada is a combination plate, not a single dish: steamed idli and a fried lentil vada served together with sambar and chutney. It exists because the two things complement each other so cleanly that South Indian tiffin counters list them as one order. The idli brings soft, mild, steamed lightness; the vada brings a savory, crisp-shelled, deep-fried counterweight. Put them on the same plate with hot sambar and coconut chutney and the meal covers soft and crunchy, plain and savory, light and rich in a single sitting. The plate is the unit here, and its quality depends on both components landing at once.
Assembly is really two preparations timed to meet on the plate. The vada in question is the medan vada: a thick, aerated urad dal batter, ground without much water and beaten light, shaped into a ring with a hole in the middle and deep-fried until the outside is deep gold and crunchy and the inside stays soft. The idli arrives fresh from the steamer alongside it. Both go onto the plate with a bowl of sambar and a chutney, often coconut, and the vada is frequently part-dunked in sambar so it drinks some up while keeping a crisp edge. Good execution serves both elements hot at the same time, the vada crunchy outside and tender inside with a clean center hole, the idli springy and fresh, the sambar hot enough to matter. Sloppy versions let one component sit and go cold or stale while the other is made, fry a vada that is greasy and raw in the middle or hard all the way through, or pair them with thin, underseasoned sambar that lets the whole plate down.
Variations are about how the plate is dressed rather than what is on it. Dunking the vada fully into sambar gives the sambar-soaked end of the spectrum, soft and spoonable; serving it dry on the side keeps maximum crunch for dipping. Sambar vada and dahi vada, the vada soaked in spiced yogurt, are close relatives that change the vada half entirely. The constant is the deliberate contrast between the steamed half and the fried half, which is the entire reason the two are ordered together.