Please show me the meanings,when to use and can it be allowed to misuse these two ? (Also greetings from VietNam!)
They are completely different. For once means for this one time. Typically it's used when something is an exception to the norm; it doesn't necessarily mean that the event being described is unique.
Say if I am habitually late to the office, but one day manage to make it in at 8 am on the dot. My colleague might remark:
Well, Verbose, you're on time for once!
This doesn't mean I have never been on time before or will never be on time in future; it simply means that my being on time is a rare event, and can in some exaggerated sense be considered unique.
At once means immediately:
Come here at once!
When I heard the good news, I called to congratulate her at once.
There is no overlap in meaning between the two phrases.