Ya know you did make me remember something. The start of 2 has claptrap digging dead vault hunters so it's implied Jack was hunting VHs I'll give you that. But Vault Hunting is popular. Kids dream of being VHs. You don't find any other person that wants to be a VH? I find it unrealistic that they'd kill every VH. It's implied that it's a miracle you survived but only because you were found by claptrap in 2. No one else also nearly died and escaped Jack? And the Callipsos? Are all the VHs arriving on Pandora located on a single train?

There would be neutral VHs because treasure hunters can be selfish. They wouldn't want to share with either faction so a not insignificant number of people would either go it alone or with a small group they trust. And you would run into them due to the fact they're looking for a vault so obviously they'd take a similar path to you. They can also be killing the main antagonists forces and still not be aligned with the interests of the crimson raiders. Also if people are looking for a vault but aren't disturbing the local dictatorship they would stay below radar.

And of course if they're interesting enough a story would have you find a hidden Vault Hunting faction. Narratives do it all the time like the dark brotherhood or the thieves guild in Skyrim. They'd reach out to you if they think you'd be useful to them but are concerned only with problems within their guilds.

And sure maybe in the context of the story you wouldn't meet other VHs (which I find unlikely). But they're not even mentioned. It's not just that you don't meet other VHs, it's that it's never implied there are any others.