Personally, I think Dante is still on edge with Vergil and here's why;
When Vergil first introduced himself to Dante, Dante was a bit suprised about the fact that he had a brother to begin with. To me when Vergil said he was looking for Dante for a long time, I almost believed it. But then Vergil explained why he needed Dante in the first place; to help defeat the demons. Dante obviously knew something was up once Vergil said that and knew he was just gonna be used. Not only that, but for there to be an organization that wants the demons dead just as Dante does seems too coencidental for Dante. And then there was what Vergil said next;
"What else were you planning on doing with your life?"
It was at that moment that Dante was officially on edge with trusting Vergil because of two reasons; The first is getting Dante to really think about his life and all that's happened. He's been fighting demons for years and was probably getting sick of it by now, so that was an easy way to get him to join, as if it was all planned out to Vergil's advantage. And the second reason was the fact that Vergil had convinced Dante so easily to join because Dante really had nothing going on in his life. It set something off in his head that made him think, "If Vergil can convience me to join him, who knows what else he could make me do, or even others...all for his plans." So Dante kept his eye on him and decided to test his trust in Vergil.
Part of this was seeing how he reacted to Kat being left behind. Vergil was willing to abandon Kat to continue on with his plans, which questioned; was he really doing all this for the humans or something more? So Dante decides to save her because he really does care for her as a friend, and also to see how vergil would react. Judging by Vergil's reluctancy, I'd say Dante was right for suspicion. The way Vergil agrees strikes Dante as something to watch out for, and you can see Dante slowly not trusting him anymore from the stare he gives Vergil after Vergil agrees to save Kat.
Finally they're at the final battle and this is where I think Dante actually believes Vergil's words of protecting each other. It's this moment where he finally puts his trust in Vergil but still has doubts about it as they go on. Thus the reason he says "We are brothers...after all." It kind of says "Okay I trust you...but just remember why I trust you."
All in all: You can call this Dante a lot of things, but blind isn't one of them. He knows something's wrong, and he knows Vergil hasn't told him everything he needs to hear about his plans, and so he must continue on with this order until he gets to the bottom of all this.
Anyway that's how I see it.^_^