Normally the FPS is taken from the project settings. Maybe you can copy&paste the voukoder logfile?
sorry i don't test enough, fifonik's answer solved my problem, it's my fault
I do not know how this is possible unless you specified VFR in Voukoder options.
VP cannot supply VFR to encoder and framerate specified in project settings is used.
You should set 60 fps and enable resample in your project settings.
after i enable resample in project setting, the output becomes real constant 60fps, thank you!