Fun fact: VTT cue identifiers are completely optional.
This file has no IDs at all. Just timestamps and text.
Some editors add them automatically. Some don't bother.
Both versions are perfectly valid according to the W3C spec.
ConvertICO's VTT viewer handles both styles without any issues.
When converting VTT to SRT, sequential numbers are added.
Because SRT requires them. Formats have opinions, apparently.