.d2v is a "pass-through file" generated by a tool called DVD2AVI. The .d2v file is basically a dummy file telling what other files should be processed and how. Typically the .d2v is opened with a TMPGEnc, as the .d2v file can be opened using TMPGEnc video encoder's so-called VFAPI mechanism. It should be noted that a file called dvd2avi.vfp is needed so the TMPGEnc knows how to process the .d2v file via its VFAPI interface -- this file is typically included within the DVD2AVI distribution package.
In its most common way, DVD2AVI creates .d2v project file that links to several .vob files and tells how to process them and when the .d2v is opened in TMPGEnc (or other tool supporting .d2v), the files TMPGEnc actually reads are the .vob files that are "passed through" the .d2v to the TMPGEnc video encoder.
In some cases, .d2v is rather non-practical, as very few tools support it. Therefor, there is a tool called VFAPI Reader Codec that basically "wraps" the .d2v file into a special version of .avi -- and as it is in .avi (even though it still works exactly like descibed above, i.e. it requires the "source" files to be left on the HDD, as the actual data comes from them), it can be opened it virtually all video processing tools, including VirtualDub, Premiere, etc.
