Vietnamese has no grammatics. Really NO. No cases, everything spoken in base form.
On the other hand it got several tones (I think 6, I never managed more than 3 of them - at least speaking them as good that the differentiation was understandable).#
Also Vietnamese includes a lot of ultra-short words, which makes them hard to learn (if “for Europeans” many words just sound the same).
So no. It is not the easiest language of the world.
On the other hand for a Vietnamese to learn a language like German is ultra-hard. I still remember the desperate questions of my (Vietnamese) wife: “Accusative ? Dative ? What even IS that nonsense! What’s it’s purpose ?”
Vietnamese basically speak like:
I going supermarket. This guy being the uncle of father (instead of saying “the father’s uncle).
No grammatical cases (not sure if I brought it good across, english is not my native language, I could more easy provide an Example in German language).
On the other hand you have stuff like this:
gà - Hühnchen
ga - Bahnhof (try going up or down in voice with a TWO LETTER WORD!!!!)
bô - Dad (actually another accent on that o - two accents on one letter - I cannot do this with a german keyboard)
bò - Beef (I actually once called by father-in-law beef, later my wife told me there is a south-vietnamese word ba I could have used which is with less “issues” - Dad-in-Law laughed, luckily he took it with humor and knew that I had not done it on purpose ;-) )