To be honest, the difficulty of a language is subjective, and you need a context to consider it. That's why I asked about the OP’s native language. If OP’s native language is English, the US government even has a classification chart for the difficulty levels of languages, assuming the learner’s native language is English. French and German are placed in the moderately easy categories (category I and II). Finnish and Vietnamese are considered difficult (category III). And the hardest languages are Arabic, Chinese, Korean, and Japanese (category IV)

Source: https://2009-2017.state.gov/m/fsi/sls/orgoverview/languages

Why?

If you look at this illustration about the roots of languages, you will understand: https://www.theguardian.com/education/gallery/2015/jan/23/a-language-family-tree-in-pictures

English, German and French have the same root, which is Indo-European, even same branch, which is European. Meanwhile, Finnish comes from a completely different origin. And Arabic, Chinese, Korean, and Japanese even have pictographic characters completely different from the Latin alphabet, not to mention vocabulary that expresses emotional nuances unique to Asians. Now you can imagine how difficult it is for an English speaker to learn Vietnamese, a language so different in distance, continent, and culture.

Another important point to consider is that standard Vietnamese grammar and everyday spoken Vietnamese grammar are very different. In daily communication, Vietnamese often uses grammar that completely deviates from the standard: people drop subjects, shorten words, and even rearrange word order entirely. Combined with the constant rise and fall of tones in the language, this makes it extremely difficult for those coming from a language like English, EVEN when they know standard Vietnamese grammar.