As you suspected, the "vaginal cuff" is the healed incision at the top of the vagina after the surgical removal of the cervix and other pelvic organs.
The vagina is just as functional as it was before surgery, but the depth is unpredictable. The tissue is somewhat elastic and stretchy–comparable to the inside of your cheek when you poke your tongue against it. If there seems to be less depth than you (and your partner) need, using dilators can gently and gradually stretch the tissue to a new capacity.
An intravaginal vibrator (like the Liv2) may be helpful to you as you determine what your needs and your new reality are. Sincere congratulations on achieving remission from the cancer for which you were treated, and best wishes as you rediscover your sexual self!