Are you using Unity 5 on Windows ?

If so, Vuforia 4.0 and 4.2 support Unity 5 32bit on Windows, and Unity 5 64bit on Mac OS.

The 64bit Windows version of Unity 5 is currently not supported.

You can download the 32bit Windows version of Unity 5 from the Unity 5 download page (select Windows - Editor 32bit)

yes Unity5 on Windows

i see, i'll try,

if i need to have a new install of the engine for the system 32bits its not usefull, are any possible to have this dll for the system 64bits?

I had a similar issue while having multiple instance of that library.

When I imported, I fiale dto remove that one and Unity simply adds the new one with an "_1" extension. As a result, the build is looking for the file without the extension and fails since it is the old 32bit version.

I would suggest to do a project search to look for any duplicate.  If so, delete the old and change the extension back to normal.