Does a monk wearing a samue break a prātimokṣa vow?
As others have pointed out, Japanese Zen buddhists (and virtually all other buddhist traditions in Japan) do not take the vinaya precepts when they ordain. In the Soto Zen tradition, we receive 16 precepts: the three treasures, the thee pure precepts (embracing moral codes, embracing beneficial action, and helping all beings achieve awakening), and the 10 major precepts (no killing, stealing, lying, sexual misconduct, intoxicants, slandering, self-praise, stinginess with the teachings, or insulting the three treasures). These are based on the Brahma Net Sutra, but are just a subset. Therefore, no vows about clothing are made.

Nonetheless, in Japan samu-e are understood to be priest's clothes and not lay clothes, although some lay people do wear them. If your vow was not to wear lay clothing, I would say wearing samu-e would not be a violation.

Also, my understanding of the Theravada vinaya is that only the three robes may be worn, and thus both Japanese and Tibetan buddhists are usually in violation of this rule.