There are hundreds of services for combining files. In the case of DVD or vob files, the list is much much smaller. I found only two services and here they are.
Using the Aconvert.com service, you can perform several actions on video files: convert, crop, merge, rotate, crop and frame offset.
You can work with: MP4, AVI, FLV, MPG, MKV, WMV, M2TS, WEBM, ASF, MOV, M4V, RM, VOB or OGV. The file size limit is 1 GB. Also, files must be of the same format, resolution and have the same frame rate.
Online Converter is a popular service for combining files of various formats.
You can use MP4, AVI, FLV, MOV, 3GP, MKV, WMV, VOB and many other formats to the input. The maximum size of one file is 200 MB.