Hi
VP 4.11 doesn't have a Constant Frame Rate option.
You can set a frame rate for your export, however this will be the maximum framerate and will not necessarily be consistant.
Under the Preset option, if you click "Detect" the framerate of your video will comeup and this value will default to the "Maximum Framerate" box.
You could try that and see if there is any difference.
In practice however, I don't think you will see a lot of differ4ence between 48 and 50 fp.
The only alternative is to download the latest version of VP 4.58
Nat