I'm going to assume you used some kind of a dedicated solution for transferring VHS to DVD and that you don't have the data files that were created from the VHS transfer.
If that's true, it leaves you no other choice than to "rip" the DVD to your computer to get a video file to work with in iMovie.
DVDs consist of highly compressed data, stored in a VIDEO_TS folder and broken into one or many .VOB files. Since iMovie is designed to work with uncompressed DV files, you will need a program to take those VOB files and convert them into a usable format for iMovie to be able to work with them.
MPEG Streamclip will do this for you, and it's free. It will only work with non-copy-protected discs (which your DVDs should be, unless the VHS work was done professionally).
Using MPEG Streamclip, insert your DVD and use the File menu >> Open DVD to access the files. Once you've finished loading them, use File menu >> Export to DV. Once all the files are converted, you can open then in iMovie and make your edits.
To burn the edited projects back to DVDs, use
Burn or Roxio's
Toast, if you don't have iDVD (which was discontinued and is no longer bundled with newer Macs).