The word rarely vacillant is quite unique and may not have many synonyms that could completely encapsulate its meaning. 'Rarely' suggests infrequent, while 'vacillant' refers to indecisiveness or wavering. Some synonyms that could come close to conveying this meaning include 'inconsistent', 'sporadic', 'uncertain', 'hesitant', 'doubtful', 'unstable', 'erratic' and 'indecisive'. However, none of these words perfectly capture the essence of 'rarely vacillant'. It suggests a deliberate or conscious attempt to avoid indecisiveness, making it rare or infrequent. This word can describe someone who is confident in their decision-making or someone who has a strong sense of conviction.