Vincristine is correct!

The location and gross appearance of this lesion are strongly suggestive of a transmissible venereal tumor. This impression is confirmed by the cytologic features, including round cells with discrete cell margins and a moderate amount of light-blue cytoplasm containing punctate vacuoles. Nuclei are round and often contain a single prominent nucleolus.

Chemotherapy is the most effective treatment modality, and vincristine is one of the most effective agents, with a cure expected in 90% of patients treated with four to six once-weekly treatments.¹ Surgery is effective for small, localized cutaneous nodules, but adequate excision is rarely possible when the tumor involves external genitalia. Radiation therapy may also be effective.

Reference
1. MacEwen G. Transmissible venereal tumor. In: Small animal clinical oncology. 3rd ed. Philadelphia, Pa: WB Saunders Co, 2001;651-655.