If VNQM doesn't meet your need you might want to check out these tools but they are pricey. They don't simulate traffic but monitor QoS on live network.
What I've done is use VNQM to monitor my call QoS and then I will load a load a link down with Ostinato for a few monitoring cycles. I get a good idea of how my network reacts to heavier load that way. While loaded at a constant rate you can also run iperf to see how qos is effecting its traffic.