So the "hymen" is the vaginal corona and it can vary in shape and thickness, it is located about 2cm into your vaginal canal so what you describe could be the vaginal corona but there's no need to worry because it won't "break" (that's a myth). You can read more about that
here.
As for the method of masturbation, I'm with Sam here. Perhaps you can go back to using your vibrator the way it was working for you for now, because the more you worry it's going to hurt, the more likely it is that you'll experience discomfort. This is just due to you being perhaps a bit tense without even realizing it, since you expect it might hurt. It's pretty common for people to prefer clitoral stimulation to penetration, or prefer using a toy than their own hands. Keep in mind partnered sex will inevitably feel different than solo manual sex, so there's only so much "preparing" we can do. Your future partner will ideally be patient and you two will be able to communicate through that first experience to avoid any discomfort.