Does a monk wearing a samue break a prātimokṣa vow? Monks and nuns take the vow of not wearing lay clothes. Zen monks sometimes wear a casual "working clothing" called a samue. It looks like a regular kimono, without the belt. Since lay people also wear it, I understand that a samue is actually lay clothes.

To distinguish the monks from lay people, the monks have to shave and to wear either the kasaya or a small version of it (the rakusu). Does a monk wearing a samue break a vow, even when he wears a kasaya or rakusu?
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.