Is it possible that a vaginal ultrasound will misdiagnose a endometrial polyp? Is it possible this is scar tissue from a fiberoil I had removed?

Polyp. Appearance of lesions on imaging is often suitable as a screening test, but as noted by the other physician's answer it could very well be something else. A biopsy will tell with certainty, as a polyp, leiomyoma, scar and hyperplasia, etc, all have very characteristic features on pathology.
Yes possible. Yes it is posssible. An ultrasound is simply a shadow like image and it does not necessarily tell you the composition of the structure. Hysteroscopy with possible biopsy may be useful in differentiating between the two.
Ultrasound is key. A scar in the endometrial lining of the uterus, where the sperm and egg implants, can occur after a procedure and could look similar to a polyp. A polyp will be bright on and the scar will be less bright . A special test where fluid is inserted into the canal called a saline hystero-salpingogram can help distingusih the 2. It can be done by a radiologist or ob-gyn. I would discuss with md.