I don't have Android Marshmallow but following apps can easily achieve the goal on Android Lollipop 5.1. Should not fail on your OS, I believe. Let me know the result in any case.

MacroDroid

When headphones are plugged:

• Trigger: Headphones Insert/Remove → Headphones Inserted


• Action: Volume Change → select Notification and Ringer, and configure the slider for the level of volume you want

When headphones are removed:

• Trigger: Headphones Insert/Remove → Headphones Removed


• Action: Volume Change → select Notification and Ringer, and keep the slider to 0

Automate

It seems to work too. Your flow should look like this:

_{(Click image to enlarge)}

Relevant blocks:

• Interface → Wired headset plugged?


• Settings → Ringer mode set

The said flow would cause the ringer and notification volume to come back to the level you set them to, before switching the phone to vibration.

Atooma

It worked as well. However, it appears to not been updated since May 2015.

When headphones are connected:

• IF: Headphone → Plugged


• DO: Audio Manager → Set ringer volume → configure the slider


• DO: Audio Manager → Notification volume → configure the slider

When headphones are removed:

• IF: Headphone → Unplugged


• DO: Silent mode → On → Vibrate: Yes

Droid Automation

It also worked well for me.

Instructions:

1. Profile: Headset plugged

   
   
   
   
   
   1. Trigger: Headset state → Becomes plugged
   
   
   2. Task: Sounds & Media → Volume: Ringtones → configure
   
   
   3. Task: Sounds & Media → Volume: Notifications → configure
   
   



2. Profile: Headset unplugged

   
   
   
   
   
   1. Trigger: Headset state → Becomes unplugged
   
   
   2. Task: Sounds & Media → Volume: Ringtones → configure to zero