I have the Vaya with discs and it is a great bike and rides very well. However, you won't get a leg up on better weight. The discs will add a little to your total too. What you might consider is building a set of wheels that are lighter, but still strong. That can make a bike feel a lot lighter than it really is. However, I do like the momentum of my Schwalbe Marathon Supremes (40mm wide) and Mavic Disc rims (719s). Once I get it rolling it feels smooth and as if I could ride all day.

Loose Chain has a good point too, but if you choose the lighter options on each of your upgrades (like a Tubus rack vs. Salsa) you may avoid an additional 5 pounds or so in the end. For really nice and light you may want to go with a Titanium frame. I'm looking at the Seven, but will take a few years to save up for that (touring frames at about $3,500). You may want to look at my The Ultimate Bike thread to get some ideas of what others are riding. Some pretty cool bikes discussed there.