I just tried it again, with the silent toggle widget, too.
Still, notifications cause vibration in silent mode, when the option to only vibrate in vibration mode is off. If it's on, then there is no vibration in silent and in normal mode, just as it sais.
My device is Samsung Galaxy S2, so not really an uncommon device
The Android version is 4.0.4.
I am currently using InoMail side by side, it has the exact same problem...