AI never had any 777-200ERs, they had 777-200LRs which were retired in April this year. The last 2 DL 77Ls too will be all gone in about 1-2 weeks.

Technically they can fly Vistara 789s to the US by blocking 4-5J seats just like how they block 6J seats on the 77W. Not sure what the engine thrust rating is at the moment and also AI seems to be prioritizing LHR, FRA and SIN flight experience hence deploying the 789s on these routes.