Hello nomk928,
Thank you for posting in the Apple Support Communities. I understand that message notifications don't vibrate as expected when your iPhone is on silent mode. I'd like to assist you today.
First, if you have an Apple Watch paired to your iPhone, notifications will go to the watch if your iPhone is locked or asleep. Learn more here: Notifications on your Apple Watch
Also, check to see if Do Not Disturb is enabled since this feature silences notifications: Set Do Not Disturb on iPhone
If there isn’t an Apple Watch paired to your device, try connecting it to another reliable network to test both issues. I also suggest checking your settings: Change notification settings on iPhone
Lastly, I recommend taking steps 1-3 here: If an app unexpectedly quits, stops responding, or won’t open. Forcing the Messages app to close, restarting your iPhone, and checking for updates often resolve many issues.
I hope this information is useful. Kind regards.