I started using Unity a couple of years ago, before vvvv.
It definitely is nice and got improved a lot over the years.
I must agree with m4d that vvvv is unbeatable in rapid prototyping.
I think Unity really has its advantages in the more realistic 3D world. It is really easy to import 3D models - setup animations etc. Doing abstract stuff like in vvvv might be hard in Unity (spreads are really neat). Also Unity is mainly used for Game development (not exclusively).
I think the disadvantage of visual programming is that (in case you dont write your own addons) your programming skills are not improved, maybe logically - but you really dont learn c# or similar.
Im agree with m4d in so many aspects, however, unity have out of the box better graphics engine than vvvv, but if you know hlsl ,this is just a programming task,
on the other hand for people who arent programmers,vvvv is very handy to develop things without a single line of code.
And of course vvvv is only runtime, and unity have the edit mode and the runtime mode, sometimes is annoying to stop the program to change just a parameter…
there are plugins for unity that allows you to use visual programming for scripting (uscript) but i didnt test how it works.
depending the project you want to do you choose the right tool.