Yea, I agree Whedon is up to the task. JARVIS has been a significant part of the Iron Man films, so Bettany playing the Vision will just be like giving a face and body to the voice. He's part of the core group of Avengers, so I'm not worried about "too many heroes". If they added more minor team members just for cameo purposes or some cliche plot device, then we would have a problem.
I mean really, if Marvel wanted to highlight a worthy character deserving of more screen time, they would have given them their own individual movie by now. Some characters just aren't popular enough to hold their own or make a sizable profit in an individual film (Black Widow and Hawkeye come to mind), but are popular enough to co-star in a team-up. Business-wise, it is too risky an investment just to appease a few fans, if it breaks even or bombs. That's just how it is.