As mentioned, 0 level spells grant you a 1st level class feature as a 1st level member of the class for 1 round. So, for instance, with a cantrip, you can gain Sneak Attack +1d6 for 1 round if you pleased.

Constant use class features are class features without a limit on their usage, like Sneak Attack, Eldritch Blast, or a Fighter's Bonus feat.

Usage class features are class features with limits on their use, like 1/day. Examples are things like Wildshape, and an Assassin's Death Attack.

I guess vestiges would count as a constant ability, as most of them don't have much restriction on their usage. But you couldn't gain the effects of more vestiges than a binder your level could bind each day.

Helpful?