Vasishtha from his desire of not obstructing this second vow (of his grandson) did not prevent him from destroying the Rakshasas.
न हि तं वारयामास वसिष्ठो रक्षसां वधात्। द्वितीयामस्य मा भाक्षं प्रतिज्ञामिति निश्चयात्॥