I don’t know. When I was in Delhi last year, we ate at the Goan canteen that caters to visitors from the state of Goa. (It was a canteen in the Goan bhawan, which is kind of like a “state embassy,” if that makes any sense.) They had pork vindaloo on the menu. They also had their own version of piri-piri, a traditional Portuguese dish, of which there are very many local variations (I particularly associate it with the southern part of Africa, particularly Mozambique, which was a Portuguese colony.)
I also saw some version of vindaloo in Mumbai.
I suppose it depends on what you mean by “authentic,” but, as far as I know, it is a dish that originated in Portuguese India. It’s based on a Portuguese dish, but it’s its own dish now. I suppose you can call it a creole. But it’s not a dish that originated in the UK and moved back across into India, so far as I know.
Here’s what seems to be a reasonable history of vindaloo. A similar dish was introduced to India via the Portuguese in the 1500s, then transformed into what is now called “vindaloo” with the addition of local ingredients and spices.
I should also add that the vindaloos in India I had were different than their UK counterparts: more fragrant, not as spicy, a bit more sweet & sour (because of the vinegar or tamarind), and the ones I had were made with pork, instead of the usual lamb or chicken I’ve seen in the UK and elsewhere, but chicken was also available.