Does a monk wearing a samue break a prātimokṣa vow?

According to Vinaya, wearing lay clothes is a minor offence for a monk / Bhikkhu. It is not so for a Bhikkhuni Sangha (nuns). The day that she wears a white dress or anything other than the robe, she is not considered a bhikkhuni anymore. But she will always be part of the Arya-Sangha, if she is one who has realized one of the 8 stages of Nobility. The type of clothes that you wear does not make you to not belong to the Noble Ones (ariya-sangha), if you have realized one of the 8 stages of Nobility.

Shaving off of head hair and beard, dressing rag-robes were common feature among sramanas of the time of the Buddha. So at the early stages sramanas were addressed as bhikkhus. Later on this became the specific designation used to denote Buddhist clergy. Among disciples of the Buddha those who entered the order are designated as bhikkhus; and the female counter-parts are bhikkhunis. The lay male and female devotees are called upasaka - male lay adherent, upasika - female layadherent respectively.

Today how many of us are aware that when we say Sangha, it is a fourfold phenomenon. Namely: 1. Bhikkhu Sangha (monks); 2. Bhikkhuni Sangha (nuns); 3. Upasaka Sangha (laymen); 4. Upasika Sangha (lay women). The Buddha's definition for Sangha is quite different and is deep in its meaning. Anybody who has realized one of the 8 stages of Nobility belongs to this noble ones.

What I read about samue is for example this:

"Samu" of samue means daily works of Zen monks such as cleaning and wood chopping.
The working costumes Zen monks wore were called samue, and it is said that the origin of "samue" is the working costume.
It used to be worn only the longer length (to the knees) of the tops without the pants.
Pants are adopted in order to make it more comfortable and active style later on.

Samue is still worn as working clothes. You can see the Zen monks wearing samue when you visit Zen temples. Because samue is originally made as working clothes, it is very functional, easy to wear and it also allows freedom of movement. Now samue is popular among ordinary people as home wear and street clothes.

So perhaps samue was a traditional uniform of Zen monks; and so, perhaps it isn't that monks are wearing lay clothes, but some lay people have started to wear what were traditionally monks' clothes.

Also, this History of the Soto Zen School writes,

The Soto school did deviate in one significant way from the Vinaya tradition as it was preserved in the public monasteries of Sung China, for it did not make use of the ten novice precepts (shami jikkai) to ordain novices or the 250 "complete precepts" (gusokukai) to ordain full-fledged monks and nuns. Instead, it followed the precedent of the Japanese Tendai school in basing even its monkish ordinations on the bodhisattva precepts (bosatsukai). In China, the traditional two-stage Buddhist ordination was practiced in part because it was required by the government, which used it to restrict the size of the sangha. In Japan, there was no single Buddhist sangha controlled by the state, and each demonination was more or less free to decide its own criteria for ordination.

So, the various prātimokṣa vows, which you may be familiar with, are not the letter of the law that's followed by the Soto Zen school.

Apparently you're right that there's a rule about clothing – it's embedded in rule number 40 of the 48 minor/secondary precepts of the Brahma Net Sutra vows:

40. Discrimination in Conferring the Precepts
    A disciple of the Buddha should not be selective and show preference in conferring the Bodhisattva precepts. Each and every person can receive the precepts -- kings, princes, high officials, Bhiksus, Bhiksunis, laymen, laywomen, libertines, prostitutes, the gods in the eighteen Brahma Heavens or the six Desire Heavens, asexual persons, bisexual persons, eunuchs, slaves, or demons and ghosts of all types. Buddhist disciples should be instructed to wear robes and sleep on cloth of a neutral color, formed by blending blue, yellow, red, black and purple dyes all together.
    The clothing of monks and nuns should, in all countries, be different from those worn by ordinary persons.
    Before someone is allowed to receive the Bodhisattva precepts, he should be asked: "have you committed any of the Cardinal Sins?" The Precept Master should not allow those who have committed such sins to receive the precepts.
    Here are the Seven Cardinal Sins: shedding the Buddha's blood, murdering an Arhat, killing one's father, killing one's mother, murdering a Dharma Teacher, murdering a Precept Master or disrupting the harmony of the Sangha.
    Except for those who have committed the Cardinal Sins, everyone can receive the Bodhisattva precepts.
    The Dharma rules of the Buddhist Order prohibit monks and nuns from bowing down before rulers, parents, relatives, demons and ghosts.
    Anyone who understands the explanations of the Precept Master can receive the Bodhisattva precepts. Therefore, if a person were to come from thirty to three hundred miles away seeking the Dharma and the Precept Master, out of meanness and anger, does not promptly confer these precepts, he commits a secondary offense.

I don't know whether Soto follows all these minor rules ... maybe it doesn't.

Wikipedia's article about the Bodhisattva Precepts says,

The Brahmajala Sutra translated by Kumārajīva (c. 400 CE) has a list of ten major and forty-eight minor Bodhisattva vows.[1] The Bodhisattva Precepts may be often called the "Brahma Net Precepts" (Chinese: 梵網戒; pinyin: Fànwǎng Jiè), particularly in Buddhist scholarship, although other sets of bodhisattva precepts may be found in other texts as well. Typically, in East Asian Mahayana traditions, only the 10 Major Precepts are considered the Bodhisattva Precepts.

And (this says 16 precepts in the Soto school, not 10 plus 48):

The Sixteen Bodhisattva Precepts in Sōtō Zen
In the Sōtō school of Zen, the founder Dōgen established a somewhat expanded version of the Bodhisattva Precepts for use by both priests and lay followers, based on both Brahma Net Sutra and other sources.

And:

Japanese traditions
In Buddhism in Japan, the "Four-Part Vinaya" was deemphasized with the rise of Saichō and the Tendai sect[7] and a new monastic community was set up exclusively using the Brahmajala Sutra's Bodhisattva Precepts. All Vinaya ordinations at the time were given at Tōdai-ji in Nara and Saichō had wanted to both undermine the power of the Nara Buddhist community and to establish a "purely Mahayana lineage",[8] and made a request to the Emperor to Later Buddhist sects, which was granted 7 days after his death in 822.

Later Buddhist sects in Japan, including the Sōtō school of Zen, Jōdo-shū and Shingon Buddhism, adopted a similar approach to their monastic communities and exclusive use of the Bodhisattva Precepts. By this time in Japan, the Vinaya lineage had all but died out and Japan's remote location made it difficult to reestablish though limited efforts by Jōkei and the Shingon Risshu revived it for a time. This was further enforced during the Meiji period, when the Nikujiku Saitai Law (肉食妻帯?) of 1872 decriminalized clerical marriage and meat-eating.