Shush has never had vibration but I get requests for it. People wish that vibration was on when volume is restored; others want the device to vibrate while it's muted.
It's all strange to me since these settings exist elsewhere on the phone. On my Nexus, there's vibrate-only between silence and volume level 1. It also has a checkbox in Settings that toggles vibration while ringing.
I've added an experimental feature that manages vibration right in the app. You pick whether the phone vibrates while muted, and after the ringer has been restored. Each option is implemented by toggling the underlying device setting.
Unfortunately this feature requires a Settings permission. I'm weighing a few options; please vote for your favorite with a +1 in the comments.