OK, technically there is a #FirstClass in Vilnius-Mockava train, but you can't purchase a seat there in connection to buying a Vilnius-Warsaw / Krakow ticket. Bought separately the price is higher.
With a combination ticket, check the class of your car, since the difference is not too visible. There are no reservations in the 2nd class, so if someone is ushering you from your seat claiming they have a reservation, you are probably in the first class. I hear.