Retained Products of Conception (RPOC): This is the most likely diagnosis given the history of a missed abortion that was actively managed. The presence of a thickened endometrium with vascularity on ultrasound is consistent with retained placental or fetal tissue, which can cause bleeding and infection.

Endometritis: Infection of the endometrium can occur after a missed abortion, especially if the management was surgical. Symptoms might include fever, pelvic pain, and abnormal vaginal discharge, in addition to the ultrasound findings.
Endometrial Hyperplasia: Although less likely in the immediate post-abortal period, endometrial hyperplasia could be considered if there were pre-existing risk factors such as unopposed estrogen exposure. However, the clinical context and acute presentation would make this less likely than RPOC or endometritis.

Malignancy (e.g., Endometrial Cancer): Although rare and less likely in the context of a recent pregnancy, endometrial cancer can present with thickened endometrium and should be considered to avoid missing a potentially life-threatening condition.
Invasive Placenta (Placenta Accreta Spectrum): If the pregnancy was not completely evacuated, remnants of placenta accreta could cause significant bleeding and have a thickened, vascular appearance on ultrasound.

Gestational Trophoblastic Disease (GTD): This includes conditions such as hydatidiform mole, which can cause a thickened, vascular endometrium. However, GTD is less common and typically presents earlier in pregnancy.
Uterine Sarcoma: A rare tumor of the uterus that could potentially present with a thickened endometrium, though this would be an uncommon diagnosis in the context provided.