Hang on
The steps here are :
1. digitalizing the analog signal from the VHS tapes. What was suggested as one possible means was to play back from a camcorder, which will ouput a digital signal in dv file format, which you could capture in iDVD. Since you don't have a camcorder...
but a DVD burner (external) isn't enough - firstly, the signal hasn't been digitalized, and a signal (even from a camcorder) has to be captured and 'prepared' ready for burning by software on the computer - and you
can't go straight from a VHS to a computer peripheral external DVD player/burner (ie it's not an actual DVD Recorder in the real sense.)
...and if you want to hear something else informative but unhelpful to you:
Depending on the quality (price), actual DVD recorders may have an S-Video input connector. If so, they would take a direct feed from an S-Video port on a VHS and the machine will digitalize it. At rock bottom, you could feed into the RCA plugs on the recorder...but you have no recorder, and the quality that way would hardly be worth it.
But you don't have/want the expense of, a DVD recorder.
Everything you're up against in what seems such a simple task you can read in
How to Convert VHS to DVD by Bob Hudson at
http://www.signvideo.com/conv-v-to-d.htm