I do not believe it is God who does the vengeance.
Let Scripture interpret Scripture:
| Genesis 4:23-24 - And Lamech said unto his wives, Adah and Zillah, Hear my voice; ye wives of Lamech, hearken unto my speech: for I have slain a man to my wounding, and a young man to my hurt. If Cain shall be avenged sevenfold, truly Lamech seventy and sevenfold. |
In the above verse, Cain is referenced regarding the vengeance that is taken by Lamech:

If Lamech does his own vengeance, then this is strong evidence that Cain does his own vengeance as well. But how can Cain seek revenge after his own death? A deadly wound that is healed would be needed.
Note that there is something extremely profound and supernatural occurring with regards to crops. Don't forget that whenever Cain tries to plant something, it either dies or doesn't grow:
| Genesis 4:12 (New International Version) - When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth. |
What is it about the Mark of Cain that kills plants or prevents plants from growing? There is more to the story here than a simple tattoo or whatever.