This is a weird topic.
1) Vector isn't an art style, but because of Flash, it apparently has become one?
2) CPU-wise, we have fucktons of processing power at hand to render stuff in Vector. Maybe 20 years ago we'd come up with bottlenecks, but we're playing games in damn 3D now, boys and girls, I think we can handle some vector art.
3) Plenty of games draw stuff in "vector" programs like illustrator and then convert it to high-res images, and just render that. This is the majority of what you are seeing when you play "high-res" 2D games like Rayman or Guacamelee.
The difference you are seeing is "pixel art" vs "vector art", but only stylistically. Pixels don't scale very well, while "vector art" most of the time is converted to a regular high-res image file, and is drawn at a much higher resolution, so looks better and is not pixelated at, say 1080p. But again, the pixel art style is to be pixelated. So...yeah.
That being said, I'm sure there exists plenty of games that render in vectors.