To answer the question posted earlier by louiscar..
MP4 is simply a container (or wrapper as you call it)
The fact the container used is MP4 has no relevance to being able to open them in Virtualdub.
Virtualdub cannot open x264 (even in an avi container) as its a AVC codec. (ISO/IEC 14496-10 - MPEG-4 Part 10)
Virtualdub only supports VFW codecs.(Directshow compatibile) (Xvid,Divx, etc)
If you want a program simillar to virtualdub to edit x264 files (in an avi/MP4 container) then i suggest avidemux
Its free and is pretty much the same as virtualdub functionality wise
http://fixounet.free.fr/avidemux/