As I said, you can merge two avi with virtualdub, but this requires that both avi are encoded with the same features (same resolution and same audio bitrate), or merging will not work.
If your standalone is not able to read AVI files, then you need to convert first avi into mpeg, and then convert the mpeg file into VOB. This is a really time consuming process and if you don’t have a powerful CPU it is much easier to buy a standalone able to read avi. In fact, the conversion from avi to mpeg can require many hours (with my computer I need about 10 hours to do this conversion). If you have a dualcore or a quadcore CPU, however, this work can be done in much lesser time
Conversion from avi to VOB can be done with many softwares, and you can find a list here
Some are free, so pick the one you like more.
Before to convert avi into vob, however, it’s better to merge both segments, but if segments were encoded with different features, the process becomes rather complicated, because you need to re-encode first one of the two segments to make it with the same features of the other segment, and this can reduce video quality. Moreover, this add some more time to the total conversion time.