Place potato chunks in a bowl of cold water with 1 teaspoon salt and 1 teaspoon white vinegar. Set aside while you prep everything else — this removes surface starch and keeps the chunks from falling apart during the long simmer.

Open the coconut milk cans without shaking. Spoon the thick coconut cream from the top of both cans into a large Dutch oven, about ¾ to 1 cup. Reserve the remaining coconut liquid.

Heat the coconut cream over medium-high heat, stirring occasionally, until bubbling and the oil visibly separates at the edges, about 5 to 6 minutes.

Add the massaman curry paste and fry in the bloomed cream, stirring constantly, until the paste darkens slightly and smells intensely fragrant, about 3 to 4 minutes. It should look dry and clumpy before liquid goes in.

Add the sliced onion, quartered shallots, and garlic. Stir to coat in the paste and cook until softened slightly, about 3 minutes.

Pour in the reserved coconut liquid and stir to combine. Add the vegetarian fish sauce, tamarind paste, coconut sugar, and salt. Stir well.

Drain the potatoes and add to the pot along with the carrot. Bring to a gentle simmer, then reduce heat to medium-low. Cover and cook, stirring gently every 5 minutes, until the potatoes are nearly fork-tender but still hold their shape, about 20 to 25 minutes.

Add the chickpeas and stir gently to combine. Simmer uncovered for 10 more minutes until the chickpeas are warmed through and the sauce is thick and glossy. Taste and adjust — more vegetarian fish sauce for salt, more tamarind if it needs brightness, more coconut sugar if it tastes sharp.

Ladle over jasmine rice and top with chopped peanuts, fresh cilantro, and fried shallots if using. Serve immediately.