Anterior vibrating lines can be recorded during impression making by:
Anterior vibrating line is an imaginary line at the junction of the attached tissue overlying the hard palate and the immediate movable tissue of the soft palate. It is always on soft palate. Due to the projection of the posterior nasal spine, the anterior vibrating line is not a straight line, but takes the shape of a cupid’s bow. It is located by:

Valsalva maneuvre—patient is asked to hold both nostrils and gently blow through the nose. This will place the soft palate inferiorly at the junction of the hard palate and the junction can then be marked.
It can also be located by instructing the patient to say ‘ah’ in short vigorous bursts.

Posterior vibrating line is an imaginary line at the junction of the aponeurosis of the tensor veli palatini muscle and the musculature of the soft palate. It is the demarcation between the part of the soft palate that has limited movement during function and the remainder of the soft palate that is markedly displaced during function. It is a slightly curved line.

It can be located by instructing the patient to say ‘ah’ in short bursts but in a normal, unexaggerated manner.

Key Concept:
Anterior vibrating lines can be recorded during impression making by Valsalva maneuver or by  Speaking “ah” in short vigorous bursts. Posterior vibrating line can be recorded by speaking “ah” in short normal and unexaggerated bursts.
Ref: Textbook of prosthodontics V Rangarajan Ed. 2nd
Valsalva maneuver
Speaking “ah” in short normal bursts
Speaking “ah” in short vigorous bursts
Both 1 and 3