You are absolutely right, you are vulnerable in therapy. Even when your therapist works to gain your trust, as she / he should, you can only know then that the therapist will do all they can to protect you from unnecessary hurt, and help you recover when you are hurting. You can get hurt during therapy in many ways, not only by mistakes the therapist could make with you, but by remembering things you forgot or never knew, or learning to see things that happened to you in a new light that is hurtful to you to realize. You may learn and grow from these new insights and understandings but initially they may hurt. So even the best therapist taking the best care of you can not shield you entirely from being vulnerable and experiencing hurt.
Maybe the only real answer to your question is to encourage you to be brave and deal with your vulnerability head on. Definitely don’t work with a therapist unless you can trust them to be both competent and caring. Know that things that are difficult and painful initially may be the very things that set you free from old alliances that aren’t worth keeping so steadfastly. Know that learning the deeper truths of your life will make it better, and that coming upon wounds that need healing is worth the work and the time. Pick your guide carefully and then take yourself, fears and all, into the fray. How wise of you to realize your vulnerability! Keep that wisdom with you and watch for all the other things you will learn as your wisdom grows. My best to you.