The second bottle of soda, still using baker's yeast, turned out as one might expect: it smelled strongly of the yeast and wasn't very good. It met the same fate as the first bottle.
The third bottle, made with champagne yeast, is not a failure. It's not a complete success either, but it is progress. There is some yeast smell, but not very much. The carbonation is good. The flavor is weak. I will be drinking this bottle, albeit slowly due to the sugar content.
I've started another bottle, this time with more of the vanilla. A stronger flavor will also help mask what little yeast smell there is. I'm staying with the vanilla and using 1/2 cup sugar until I think I have the flavoring amount right. Then I'll see about easing back on the sugar (one source claims that less than 1/4 cup will do for a 2L bottle) and other flavors.
I made another loaf of bread and that also sort of turned out. It's not bad, but it is a bit more dense than I'd like. I probably didn't let it rise quite enough. I also probably haven't managed to judge the right amount of flour or water to get the dough just right. Still, it's not a brick, it's just a more dense loaf.