I would like to set the watch alarm mode to vibrate only.

Hi

When Silent Mode is enabled, alarms and timers are muted (unless your Apple Watch is on the charger and connected to power), but still provide haptic alerts.

Follow the instructions here to enable and disable Silent Mode (note that other notification alerts will also be affected):

Adjust brightness, text size, sounds, and haptics

If your Apple Watch is not providing haptic alerts for alarms, then the following steps may help:

Check your settings:

• On your iPhone, in the Watch app, go to: My Watch > Sounds & Haptics.
• Check that Haptic Strength is set to the middle or far right hand side of the slider.

If you have not yet done so, update your iPhone and Apple Watch to the latest software, starting with your iPhone:

• Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support
• Update the software on your Apple Watch - Apple Support

Restart both your iPhone and your Apple Watch, turning both devices off together, then restarting your iPhone first:

• Restart your iPhone, iPad, or iPod touch - Apple Support
• Restart your Apple Watch - Apple Support

If the issue persists, then it may help to unpair and re-pair your watch:

• The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app. Choose to Restore from Backup during set up. Most data and settings will be restored, with some exceptions (eg Apple Pay cards, passcode).
• Unpair your Apple Watch and iPhone - Apple Support
• Set up your Apple Watch - Apple Support

I have the same issue. When watch in silence mode the alarm will trigger but with no vibration. I’ve done all the steps here and it did not solve the issue. Envíen a complete reset without recovering from backup worked. I think it has to do with the newest watchOS 4.0 my watch is series 1.

Under watchOS 4, the following steps may help, having resolved the same issue for other users:

• Create a new alarm on your iPhone, ensuring vibration is enabled for that alarm.
• Create a new alarm on your Apple Watch (the iPhone alarm can then be deleted).

You're most welcome! 🙂

Thank you, thank you, thank you!

For some illogical reason this remedy actually does the trick! Anyone with Series 1 watch who recently updated to os 4 should do this to regain haptic alarm, which Apple somehow managed to disable.