A method to get all the players in a group (in the form of an ArrayList) would be great.

String groupname = "admins";
permission.getPlayersInGroup(groupname);

this has been requested more than once, permissions systems simply don't support it.

And what's stopping you, the plugin author, from doing that internally in your own plugin?

Could you not loop through all offline/online players and all groups, and check if their primary group is the same as the group you're looping through to compile a list of players in that specific one?

Vault doesn't attempt to provide compatibility for things that the permissions systems themselves don't already support due to the high amount of data that would need to be managed inside of Vault. As turt2live mentioned, if you think Vault could do it somehow then there's no reason you can't simply implement it in your plugin. Vault's intention is never to cache or perform any form of Data storage/management at all.