I've had the same problem. I was using the SL viewer for a few hours yesterday. I went off and when i came back on, I was given the new viewer by LL in a download. When I tried to open it, it just closed down while the message: "Initializing VFS" was showing. I tried so many things to get the viewer to open, including deleting and reinstalling it, manually cleaning the cache, and completely deleting everything, including hidden files. I did live chat with Linden support and basically was told to do all the things I'd already tried and I did them again, with the tech support person.
I downloaded and updated my video driver for nvidia after I was asked to do that by LL support. It didn't help. BTW, everything else I do on my pc was working normally and as I've said above, the SL viewer had worked normally that morning.
I asked the support person if anyone else was having this problem with the browser update and I was told, no, you are the first.
Obviously, this is a problem with the update and not a sudden problem with my pc. It's not just some dumb coincidence for all of the people with this sudden problem as soon as the update was sent out.
I'm not happy that I spent hours trying to fix this. It was very stressful and I don't believe that it has anything to do with my pc at all.
I still can't get the SL viewer to work, but the good news is that I tried the Phoenix viewer for the first time and it works. I'd rather use the SL viewer because I'm used to it and I have a Premium account that includes support. But that support failed. If Linden Labs fixes this, I'd probably go back to the SL viewer, but I'm using Phoenix for now...once actually so far, and it's very much the same, just more complicated. Without it, I'd be cut off from my SL friends and my nice house on the Mainland.