Set alarm quiet-ish on your phone, put it under your pillow then hope the volume is loud enough to wake you, but the muffled sound doesn't wake them up?
Perhaps the vibrating option is not a good option, but a low volume alarm could sound loud enough for the person on the pillow, and substantial quieter for someone only a short distance away.
It's like an alarm playing through headphones only without having to wear uncomfortable headphones while you are trying to sleep.
They could try different alarm tones too. If I go with something harsh and irritating I wake up angry about it, if I co with something that brings me around gently I'm as happy as any man getting up at 4:15am.