I'm interested in the interaction between you and the server!
I've never had any problems with ordering vegan at Indian restaurants. I'd have guessed the plates got switched.
I would have repeated that I ordered the vegan mango dish and had it returned.
I wouldn't be negative unless they come across that way. It's no different than any other dish that came out wrong. It happens
I wound up having a discussion with the server and later the manager. The manager blamed the server for the mistake, adding that "She's really smart, she has a PhD", and the server apologized without explaining why the mistake was made, or who actually made it. It's not like there was an "add a protein" option on the menu for this dish, so whether the mistake was the server's or the chef's, they went off the menu to come up with this! The tiny "complimentary" desert (her words), which I think every customer gets as it was brought to me without any consultation, didn't help, as it wasn't vegan either (I had to ask a few questions to arrive at that conclusion) and it was politely refused.
They did replace the food with the proper dish. As a 20+ year vegetarian, poking around in my food to determine if there's animal flesh in it is not something I wish to experience. The manager gave me a generic, "How was everything?" near the end of the meal, making no mention of the error. I gave a half-hearted "okay" and he walked away. For whatever reason, he came back a few minutes later and asked why it was "only okay". I explained why my experience was pretty much unsalvageable after the mistake and he just stared at me. I tried to calmly and politely explain that an animal had died for my meal, whether I ate that animal or not. At that point, he literally walked away, mid-sentence. So I added, I thought sufficiently under my breath and for my own entertainment, "Yeah, you really don't give a s**t, do you?". I paid the bill, I gave the server a standard tip, and left. The manager followed me into the parking lot, and offered me another half-hearted apology (I guess realizing he'd been rude before? This is when he added the weird comment about the server being smart.) He still didn't get why replacing the incorrect dish wasn't enough to make me happy.
I was visiting from out of state, so it's unlikely I would have returned to this restaurant (now it's a certainty). As I drove to the concert venue I tried to see his perspective - for a business that has fridges loaded with dead animals, what's one more dead animal? We made a mistake and we fixed it, get over it. It's like we were just speaking two different languages.