Well then it is the file(s) to blame, not the VDub. What you can do is join them in the program, then save the audio to a .wav file. Then save the video using direct stream copy with no audio to .avi. You can now use a sound editor to either stretch the audio starting where it goes out of sync, or instead, add or remove time to accomplish the same thing. It all depends on if it goes out of sync gradually or at one instant, it becomes off by the same length throughout the rest of the file.
I recommend Sound Forge 9.0e because you can open the video and audio as separate and see what's going on as you edit.
Post back if you need any help. Good luck.
You could also try first removing a tiny bit from the beginning of the 5th vid before joining, using direct stream copy for both audio and video. The add all as you tried. Maybe that will make a completely synced file. Good chances.