You’re dealing with two different aspect ratio’s.
640x480 is 4:3 aspect ratio (in the picture below, green is 4:3)
1280x720 is 16:9 aspect ratio. (blue color)
To have no border, you have two choices.
- Crop top & bottom of the 4:3. (maintains the 16:9 ratio)
- Crop left & right of 16:9. (maintains the 4:3 ratio)
You could add a decorative border around the 4:3 to keep the 16:9.
You could have the 4:3 all the way to the right with some sort of picture or text on the left.
Going off on a tangent here… Have video in 16:9, say of waves crashing, have that be your border, then put in the 4:3 video.