For prosperity and future users I'll post the pin out of the Vic-20 here. It's true the 5-din C64 plug will work, and the pin out is almost the same. I use the same home-made cord for both my Vic-20 and C64.
The parts are easilty available, and you can make a new cord with 5 minutes, a soldering iron, and a old RCA cord (if you don't care that the video line isn't as insulated).
Pretend this is a circular MALE 5 din plug... (You'd be looking at it with the pins away from you (i.e. from behind, or if you will, if you were looking directly into the port in the computer.)
Code:
[3]      [1]
  [5]  [4]
     [2]
1. +6V (don't put anything here!)
2. Ground
3. Audio
4. Video Low
5. Video High

I'm not sure what the difference between Video Low and Video High are.... I just know that pin 4 is Video out on the C64 specs and it works for me. YMMV.

For comparison purposes, the C64 5-Din pin-out is:
1. Luminence
2. Ground
3. Audio
4. Video Out
5. Audio In

Sté