Well how old is Namek? I'm not too clear on that myself. And I'm not saying Goku could win, because there's no doubt he could, I'm just saying there's probably a large number of realities where he didn't or couldn't. Like how there's probably a large number of realities where he was easily triumphant.
True, but if you recall Vex had already scouted Venus [i]billions [/i]of years ago. My assumption is they had no real reason to act until humans started to move in on their turf and were also spurred on by the Darkness.
If I had to guess, the Vex left Earth alone because it had to much water, or showed evidence that the planet would maintain to much of it.