Aww.
We all make mistakes like that from time to time though, so don't beat yourself. Now you know. And yes, the "blood ring" description is confusing because a viable egg always looks like it has a ring around the outside of the developing "placenta." And yes, vasculature almost always means health egg--the blood vessels deteriorate VERY quickly when the chick dies.
In response to the question about why get rid of bad eggs, there is the exploding egg problem, but even more importantly, if you have a large number of bad eggs, they will off-gas and damage the air cell development of the healthy eggs. I learned this the hard way by losing all but four of 21 healthy eggs because of 20 bad eggs that were in with them. Here's what happened: I had healthy duck eggs in the incubator, and added 60 quail eggs, some of which I knew were likely to be bad, but I didn't worry about it because quail eggs are tiny and I didn't think an exploding one would do much damage.
What I didn't realize is that the bad quail eggs were off-gassing moisture that was affecting the humidity in the incubator, and as a result, the air cells in my healthy duck eggs were not developing. When it came time to hatch, all but four had small air cells and drowned in the shell after pipping internally. It was awful carnage--all those perfect, healthy babies dead in their shells.
That having been said, I don't toss eggs until I'm absolutely sure they're dead, usually after two candlings at least four or five days apart. I mark them the first time if I think they're bad, and then check them again the second time. If it's just a few eggs, they don't produce enough moisture to make a big difference. But I would definitely want to know well in advance if I had 20 bad ones in the incubator again!
Unfortunately, quail eggs don't candle very well, so I just make sure I'm always putting VERY fresh eggs in, and hope for the best. I also regularly check air cell development so I'll know ahead of time if there's a problem and can adjust appropriately.
Good luck with the rest of your hatch--