If the files are exactly the same try Pecks Power Join for .avi (like in 1of2 & 2of2 or files that you made yourself with the exact same settings.)

http://www.digital-digest.com/

For .vob files I'm not really sure what editing software is good

If you want some really good free all round software for editing .avi try VirtualDubMOD. It's really good and gives you lots of advanced features. It's also very complicated

Use Google to find it and guides for it, if you're interested