In a possible sign of a new trend in Southeast Asia, economic pressures are driving smallholder shrimp farmers from Vietnam's Mekong Delta across the Cambodian border in search of new land. Building from ethnographic research with Vietnamese shrimp farmers in Kampot province, Cambodia, this paper explores the structures, mechanisms and relations that facilitate and impede the ability of Vietnamese migrants to gain and maintain access to land in Cambodia. The Vietnamese migrants in our study bring capital and farming skills, but their ambiguous legal status and their lack of social networks and experience with the terms of access in Cambodia render them vulnerable to exclusion and dependent on a local broker to mediate their interactions with landowners and authorities. We recount the migrants' attempts to overcome the uncertainty of their mediated access by bypassing the broker and cultivating direct social ties with Khmer villagers, border authorities and the landowners themselves. This study generates new insights into the dynamics of cross‐border livelihoods in mainland Southeast Asia and more broadly illuminates the central importance of migrant–broker relationships and migrant agency in seeking to overcome dependency on brokers by forging new social relations in border areas.