This app was mentioned in 2 comments, with an average of 3.50 upvotes
https://play.google.com/store/apps/details?id=com.stastnj1.discreettimer
I found this app which does what you want I think and you can deactivate the silent notifications and all it does is just vibrate once the time is up.
Maybe an app like this would work? https://play.google.com/store/apps/details?id=com.stastnj1.discreettimer