Hello and welcome to the forum
I would say that if you cannot see the borders in VDub2, then there are no borders.
Maybe its just the settings that you are using in PotPlayer [I dont gonna mention WMP].
Suggest Settings/AspectRatio/
Keep Aspect Ratio(Recommended), and
Default Display AR (Recommended)
and as it plays, hit
CTRL/F5 to cycle around the half dozen possibilities.
If further probs, post a sample for the guys. [also your PotPlayer Aspect Ratio Settings]
(Please say if no longer a problem)
Dont know why you would want it in AVI (Maybe for your Old MainFrame running W10 21H2
).