Vision only came after Scarlet Witch though. He didn't arrive until sometime later, whereas Wanda was there already from about issue 16 just with her brother.
We don't need Vision yet to have Scarlet Witch. And besides, she wasn't always with him. Later on she had some dalliances with Wonder Man.
Hopefully they won't go the Ultimates route though and have her in an incestuous relationship with Pietro.