Frankly I don't see why vbulliten is so much more preferred? I haven't used it but the only thing I can see that I like more than with phpbb boards is the way they handle PM's, which has gotten worse with VB3.0 anyhow.
Also, just about all vbullitens boards look all the same plain type of style whereas people who use phpbb usually have more unique looking boards.
As for invision, just don't like the way it looks, not sure of it's functionality since I havem't used it either... I also think the "pinned" is very lame & they should have stuck with "sticky".