Does practicing masturbation invalidate the vow made by the questioner to stop it, if things are fine and he does not go for medical examinations, knowing that he does not remember the exact wording of the vow?

Masturbation is forbidden (haram) and must be abandoned. The questioner's vow to abandon it is still valid because he conditioned it on the results of medical tests that have not yet been conducted. In case of doubt regarding the wording of a vow, nothing is binding on him, as the default is acquittal of responsibility. Whoever vows to abandon a sin and then commits it must offer an expiation for breaking an oath. One must be wary of obsessive thoughts (waswas) and treat them by ignoring them and frequently remembering Allah, while also considering consulting a specialized doctor.