The flows of diatomic gas of anharmonic oscillators in expanding nozzles with different geometry are investigated. Nonequilibrium vibrational distributions, which may realize in different cross sections of nozzles, are obtained by means of the methods of kinetic theory. These distributions are the generalisations of the Treanor distribution, they contain the complementary additive invariant which is equal to vibrational level number only in the lower part of molecular vibrational spectrum. It is shown that in some nonequilibrium conditions the gasdynamic systems include besides the conservation equations of mass, impulse and energy one more equation for summary value of complementary invariant. These systems are solved in quasi-one dimension approach. The results of the calculations of gasdynamic parameters and vibrational populations for the flows of nitrogen molecules in expanding nozzles are presented. The influence of the nozzle form on nonequilibrium vibrational distributions is investigated.