Measure object planar displacement on slight moving or vibrating plane
...turns out that the plane might move/vibrate slightly (~ +-5mm), would I still getting good results? 


I do not see why not. And this is making reasonable assumptions about the equipment.

You have a monocular camera, tracking motion from a viewpoint that is vertical to the direction of motion.

Now, if the plane moves purely in the Z direction (that is, on an axis that is vertical to the lens), then the only thing that will change is the size of the object.

The problem this might create is in the Signal To Noise ratio of the tracking. In other words, if you are tracking a marker against a background and the marker's size "pulsates", then the tracking estimate of the frames where the target is relatively larger will be better (within reason).

By how much will your marker / object size change, depends on the focal length of your lens.

If the plane does not move purely in the Z direction but sort of oscillates back and forth as if it was a pendulum, then it introduces an additional perspective error to the measurement that is proportional to $\sin(\theta)$ where $\theta$ is the angle of oscillation.

In other words, a monocular camera imaging a vertical line segment cannot know if the imaged size is true or the projection of a much larger vertical line segment that stands at some angle $\theta$ towards the lens.

In general however, the "magnification error" (first case) grows smaller with the length of the lens (lower in long lenses) and vice versa. There of course, you have to balance the field of view too so that it includes the variation you are trying to measure.

If the equipment you are using has specific characteristics regarding focus and image formation, you are going to have to take those into account of course.

Hope this helps.