Vanish.

I read that Vanish increases the power of Shadow Partner.

Is this true, and if so by how much?

It only increases the damage dealt on the first attack after canceling dark sight by the % it shows on the description.

Would it be worth it to get it and leave Claw Booster at six?

All depends if you care about casting booster every 60 seconds, if you do you should only add 1 point for its dark sight canceling capabilities.
I maxed it on mine and didn't regret it.

I'm pretty convinced to max Booster if possible before even considering adding any more than 1 point into Vanish, speaking from personal experience.

It adds 30% to your Shadow Partner damage. Example:

20.635 / 2 = 10.317 damage without Vanish
10.317 * 1.3 = 13.412 damage with Vanish



Anyway, you can max both Booster or Vanish at lvl 110+ Since noone maxes Avenger, Sweb or Disorder.