I have a single ticket from Kolkata, India to Amsterdam. The first part consists of a domestic flight with Vistara from Kolkata to Delhi and the next is with KLM from Delhi to Amsterdam. I was told by Vistara officials that I'd have to check in my baggage again at Delhi airport due to immigration. My question is whether I'd have to pay for excess baggage at both the airports or paying once at Kolkata would suffice? I tried to contact both the airlines regarding this but received no answers.
Vistara has an interline baggage allowance page according to which (emphasis added using bold italics)
If connection to/from international flight on one single document/ticket:
For customers travelling only with Vistara from domestic origin to international destination or vice versa on a single document (that is, a single ticket mentions the origin point and the final destination including any transit points), will be offered the same baggage allowance end-to-end as mentioned on their International ticket, regardless of whether it is on weight or as per piece concept as long as all the connections are within 48 hours.
If any connection is beyond 48 hours, then the baggage allowance for the corresponding sector will apply as shown on the document, irrespective of the rest of the itinerary.
Passengers travelling on Vistara, to and from international destinations with our interline/codeshare partners on a single document (that is, a single ticket mentions the origin point and the final destination including any transit points), will be offered the same baggage allowance end-to-end as mentioned on their International ticket, regardless of whether it is on weight or as per piece concept as long as all the connections are within 24 hours. Post 24 hrs the baggage allowance for the corresponding sector will apply as shown on the document, irrespective of the rest of the itinerary.
If connection to/from international flight on separate document/ticket:
The baggage allowance shown on the ticket for each the corresponding leg of the journey will apply i.e. the baggage allowance mentioned on the ticket for the Vistara segment would be applicable for that segment.
Any weight(s) in excess of the aforementioned allowances shall be charged as excess baggage. Vistara will charge the customer for the excess baggage at the time of check-in and bag(s) will be check-in for the Vistara segment only.
From the page formatting it is not clear whether the emphasized portion is for separate tickets or for all cases, but language (aformentioned allowances) seems to point that this is applicable to all tickets.
From this, IMO if you have excess baggage Vistara will only check your bags in for domestic sector (after you pay excess baggage fee at domestic rates) & then you'll have to check them in with KLM at Delhi (after paying KLM's international excess baggage rate).
What the airlines officials told you seems to be correct except that it is not immigration but their policy which forces you to check bags in again at Delhi. If you had no excess baggage then my guess is they could have checked your luggage through to final destination because KLM is an interline partner & Vistara press release had mentioned through check-in for KLM.
If you are interested IATA has an interline document which states (p10) that excess baggage is particularly tricky & requires additional agreements between airlines. Might explain why Vistara handles excess baggage on per sector basis.
Generally you pay excess fees per way for each ticket. So if you have a return ticket A->B->C and C->B->A then you pay excess baggage fees when you check-in for A->B->C. Even if you have to transfer luggage when connecting for the B->C segment, the extra allowance is already paid for. Your bag already be tagged to your final destination or you have to get it at the check-in counter for the B->C segment at which point you should show them the receipt for excess baggage paid, should they not have access to that information.
You have to repeat the process in reverse for you return flight. There could be slightly different agreements in place between airlines which is why contacting them was the right to do. If you tried online contact, you may have better luck calling. Usually though, you are not expected to pay excess fees per segment.