Why is the origin (patient zero) required to cure a virus?
Nope. This is a total movie fallacy/exaggeration.

How worthwhile it would be to isolate the original strain is debatable, but certainly not necessary for vaccinology purposes.

On one hand, the inital strain might be interesting to study if its an aetiological agent which has never caused human disease before. It would be the first 'snapshot' of the genomics/transcriptomics of an organism/agent which has been able to jump a species barrier perhaps, or has evolved to survive at our body temperature, evade our immunity and so on.

However, as the infection progresses through a population, in order for it to stay infectious, it must by definition have maintained the molecular patterns that permitted it in the first place, so isolation of the 'zero-th', 10th or even 10,000th infection shouldn't make any real difference from a therapeutic standpoint. It may even be the case that as subsequent infection has in effect, passaged, the organism, that the later strains may even be more dangerous than the initial one.