Hi Emil, the gentle vibrations start with a single vibe, and after 30 seconds, 20 seconds, etc., other vibes follow. After a while, the vibrations become continual. If you wish to have some buffer time to wake up only with the vibrations, maybe choose longer period for Wear OS (and Watch One) platform, so the vibrations have enough time to enfold.
If you only have one vibe, and nothing follows even after some time, please check if you have “allow background activity” enabled for Sleep app - Galaxy Wear > Watch settings > Apps > Sleep tracking > Allow background activity .