basically vvvv uses whatever windows provides, so your questions is a generic “how to play videos on windows” question (with all the complications which arise).
if your video plays and scrubs fine in windows, vvvv can do similarily - just connect the SeekPosition and DoSeek pins on the FileStream node.
mjpeg is just one possible codec for the avi file format. it is not provided with a standard windows install - you need to buy the moonlight elecard decoder. with that codec installed, using it is as easy as any other avi file - you can use basically any program dealing with avi files to recompress your movies (virtualdub, vegas, premiere, aftereffects, whatever).
note that v4 can also deal with quicktime, so you can use quicktime codecs as well. from what i know there are some standard quicktime codecs quite capable of scrubbing.
also note that v4 will always stream videos from the hard disk, so if you want to have blazingly fast 60fps scrubbing experiences see the “looping” thread in FAQ AV