Final answer:
Tumor size and depth of invasion are the most important prognostic factors for SCC of the vagina.
Step-by-step explanation:
The most important prognostic factors for SCC of the vagina are tumor size and depth of invasion. These factors help determine the extent and aggressiveness of the cancer, which can affect the prognosis and treatment decisions. For example, a larger tumor size or deeper invasion may indicate a more advanced stage of cancer and may require more aggressive treatment.