I would have thought that AI would have no problems interlining separate tickets on their own reservations (AI-AI)?

The other option is connect to vistara 1hr15 later than AI but I feel that’s more risky in terms of checking in without a visa?

TIMATIC says this so it does seem pretty clear that I need them on the same ticket 😬:

TWOV (Transit Without Visa)

Passengers transiting through Bangalore (BLR), Chennai (MAA), Delhi (DEL), Kolkata (CCU) or Mumbai (BOM) with a confirmed onward ticket for a flight to a third country within 24 hours. The connecting flight must be booked on the same ticket and the luggage must be checked to the final destination.
They must stay in the international transit area of the airport and have documents required for the next destination.
Passengers transiting through Hyderabad (HYD) with a confirmed onward ticket for a flight to a third country within 24 hours. They must stay in the international transit area of the airport and have documents required for the next destination. Airlines must obtain the approval from the Immigration before departure.
Hotel facilities are available in Terminal 3 at Delhi (DEL) for passengers with a boarding pass for their connecting flight.
Passengers on flight AI 263 transiting from New Delhi (DEL) to Thiruvananthapuram (TRV) and traveling to Male (MLE) can transit without a visa. They must remain in the aircraft in Thiruvananthapuram (TRV).