The only real, genuine enforced code is jacket for men at V&A, and no swim suits/tank tops (on men) at all resort hotel restaurants. I'm told that also applies at Bistro, but I can't say for sure. You may be told "business casual", or "smart casual" when you make a PS, but this is a recommended mode of dress--it's NOT a requirement.
Bottom line-you'll see all manner of dress at these restaurants. My collective experience over many years of dining seems to be about 70% "business casual" and 30% "theme park" casual on most patrons. We've done all the "fine dining" spots in walking shorts and Disney T's and no one has even given us a second glance.