I did a lot of tests, and meanwhile read a lot of things about x264 encoding; it seems that final bitrate using --vbv-maxrate is near perfect - maybe around +/- 0.2%!

So, I've gone back to my old projects, checked their bitrate, and they are fine; the conclusion is, after I was forced to reinstall from scratch the OS, the problem arised... still, I have no idea what could cause it, as I used different x264 executables...

Last chance is to take another HD, install (again) the OS from scratch, install avisynth, install x264 (and nothing more), then try to make the same encoding, using the same settings, and see if it works - it SHOULD work that way! It will take a lot of time, but I can't loose sleep anymore! (^^,)

Any advice is really appreciated, though!