There is little need to convert between the two media formats, AVI to VOB, and a direct conversion between the two isn't possible using most media conversion software. VOB (or video object) files are only designed to be played on a video DVD, while AVI is designed to be played on a computer.
Most forms of AVI conversion software can convert AVI to other popular media formats that are used by computers. These formats include WMV, MOV, and variations of the MPEG format. While there are many media conversion freeware programs that can do that, not as many offer an option to convert AVI to VOB.
There are many limitations to the VOB format that makes the need to convert AVI to VOB less than practical. While an AVI file can be many gigabytes in size, a VOB file cannot exceed one gigabyte. So if the AVI is too large, it is impossible to convert the movie into a viable VOB file without first splitting it. Furthermore, the use for a single VOB file on its own is relatively limited. While a few media players support independent VOB playback, most do not.
And while a single VOB file will play in select media players, it cannot be burned to a DVD that will work in most DVD players. VOB files are used in conjunction with IFO and BUP files, other files that tell the DVD player how to read the VOB files. These files must be placed in the proper directory, and have the correct file names, in order to work.
In most cases, the easiest way to convert AVI to VOB is to download or purchase a DVD burning program. Many programs that can create DVDs from AVI files also give you the option to save a backup of the DVD to a hard drive. These backup files are usually in the form of VOB files, with the corresponding IFO and BUP files that will allow them to play properly.
Before trying out any program that claims to convert AVI to VOB or backup DVDs to your hard drive, do some research. Many of these programs, especially the free ones, come packed with adware and spyware. Not only that, many of them don't even work as advertised. Compare products, read reviews, and visit forums. If a program has a trial period, take advantage of it. If you do all this, you increase your chances of finding the right program that can convert AVI to VOB.