I have a flight soon from VIE-AMS-BLR. The VIE-AMS part is operated by KLM, but the second leg is operated by Jet, who recently suspended all international flights. My ticket is from KLM, so who should I contact for this? And since flight tickets are going to be very expensive, is it likely that I will get any form of rerouting?
It do not matter who operate the flight. If your ticket is bought from KLM, they should reroute you, and eventually compensate you (but if the flight were scheduled after 7 or 14 days the cancellation, the compensation rules changes a bit).
I suggest you to go to KLM site and register yourself and your flight, so that KLM could contact you in case of changes, and possibly you will see the option you can have. [Good to have some ideas of alternatives before contacting an airline].
In any case you should also contact KLM (or the company which issued you the ticket).