Which tones are hardest, why, and where to start.
Vietnamese speakers' difficulty concentrates on Tone 4 — the opposite of Western learners. The specific direction is T4 produced as T1, especially when two Tone 4 syllables are adjacent. Vietnamese ngang has both high-level and high-falling variants, so Mandarin T1 and T4 assimilate into a single Vietnamese category. Tone 3 sandhi is also under-applied (the first T3 of a T3+T3 sequence is not raised to T2).
Source: Tu, Jung-Yueh (2023), Production of Mandarin Disyllabic Tones by Vietnamese Speakers, ICPhS 2023, Prague, paper ID 48 | 政大硕士论文《越南语母语者普通话声调偏误研究》(45 名岘港大学中文专业学生) https://zenodo.org/records/15549791 | 《臺灣華語教學研究》第 21 期
Group averages tell you where to look, not what you actually do. Two speakers of the same first language can differ a lot. The only way to know is to record yourself and compare.
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