I just got a Sony KDL52W4100 and love it, so I'm totally biased.
Having said that, I was looking at the same sets (except the Vizio). What did it for me was seeing a 52A650 next to a 52V4100. The Sony looked better in every way. Blacker blacks, whiter whites, more vibrant colors and a nice, sharp HD picture. Checked out the 46"ers and the same story.
So I was all set to get the V but the place I went to didn't have any in stock so I ended up getting a crazy deal on a W.
I will say the Samsungs had a good picture but the Sony's was just better, IMO. You really can't go wrong either way.
Don't know anything about Vizio so I can't help you out there.