I've tried o change the capture frame rate in Virtualdub (v1.9.11, 64bit) from PAL (25.0000) to NTSC (29.9700):
No matter which value I've entered, after closing and re-opening the dialog, the framerate was back to 25fps.Capture > Settings > Frame rate
I've even tried to "round to nearest millisecond", which changed the value to "30.3030", but closed and re-opened it and: back to 25fps.
[SOLUTION]
The input signal from my capture card was set to deliver PAL 720x576 @25fps.
Here's a HowTo for switching between PAL and NTSC capture in Virtualdub. The solution mentioned there is to check and edit the settings in Capture mode's menu:
Video > Capture filter > Video format