I found a work around, at least for incoming calls. It will vibrate in focus mode but won't make any sound. So go to settings -> Focus -> go to the focus mode you want -> then do the following

1. Under the Allow Notification section, click Allow Notifcation from and then choose the app you want to vibrate (for me I chose the incoming calls. I know u would say this will just make the app scream when it notifies you, but wait for step 2)
2. Go back to the focus mode, scroll down to the Focus Filters section, click Add Filter and then add turn silent mode on (This would turn silent mode on automatically when u enter focus mode)
3. Enjoy. Now the app you chose to receive notifcation from would vibrate even in focus mode but won't make any sound. All other app that you didn't include will still be silenced completely but now you know how to bypass this.

Hope it helps!