Breast cancer is the most common tumor among women, representing the second cause of cancer deaths in women. Treatment with chemotherapy negatively interferes with nutritional status. The intake of vitamins before, during and after treatment in a pilot cohort of women with non-invasive breast cancer (type I, II) treated at the Valencian Institute of Oncology (IVO) is evaluated. A 3-day anthropometric and nutritional assessment was performed using the DIAL program. Nutritional intake is compared with the values of Estimated Average Requirements (EAR) and Dietary Reference Intake (DRI) provided by the United States Department of Agriculture (USDA) and the European Food Safety Authority (EFSA). There is an overall decrease in vitamin intake during treatment which worsens at the end of said treatment. The decrease is significant in the case of vitamins B2 (p = 0.006), B3 (p = 0.042), B5 (p = 0.001), and B8 (p = 0.021). The relative risk during and after treatment increases with respect to the reference timeframe, before treatment. Deficit risks are statistically significant in the case of vitamins B5 (p = 0.001), B8 (p = 0.001) and B12 (p = 0.001). Decreased vitamin intake during treatment suggests a negative change in the patients’ dietary behaviors during this time. Nutritional intervention and support may be beneficial to optimize overall dietary intake and maintain compliance with EAR and DRI for patients during a time in which adequate nutrition is important.