1Password 8, is there not longer an open to default vault anymore?

I understand your confusion as so many features from 1PW7 have changed, disappeared or work differently as they've been moved to 1PW8. Frustrating that there is no documentation on where features went.

In this case, 1PW8 has "Collections" where you define what vaults belong to a collection. Once you define the collection (use the drop down by your vault icon), you can set it as the default "collection vault".