Could you use ultrasonic vibrations instead of a roller with an SLS/SLM printer?
No
The problem is twofold. Resonance and Granular convection
Resonance
Let's start with an empty box. We toss in some powder to create the first layer and use an ultrasonic to create a first layer. What happens? The bed starts to resonate depending on the sound you send into it in patterns - and the powder starts to form valleys and ridges along them as one can see in this video.
Granular Convection
What happens if one shakes a box of fine granulate that contains larger items? Granular convection happens! All items raise simultaneously and the small items start to fall first, resulting in them getting under the larger ones, so as a result end up pushing the large items up.
Because of both effects, there won't be an even layer and it would raise the items printed, even if we managed to get good layers.