Thank you for the post.
The average vaginal length when not aroused is around 3-4 inches. So not that long. It does elongate with arousal, which ensures for most women comfortable penetration.
Have you had a routine pelvic exam with a gynecological practitioner? They should be able to give you feedback on the positioning of your uterus and cervix.
Even though you can feel your cervix with your fingers only a few inches in, a penis could be different.
Sometimes changing the angle of penetration can help insert further into the vagina.
I think using a vibrator and practicing penetration is a great idea. I would try insertions in different positions and angles and see if that makes a difference.