Hi Trinh, 1. 'Thanks Lisa' is correct. if you want to thank her. 'Thank Lisa' is correct if it is an instruction to thank her. 2. Have you gone implies an expectation. "Have you gone to Vietnam"? (Someone thinks you may have gone to Vietnam) Have you been implies no expectation. "Have you been to Vietnam"? (Someone does not know if you went to Vietnam)

1) If you are talk you Lisa, you would say "thank you Lisa" If you are talking to someone else about something Lisa did for you, you would say "thanks TO Lisa, ... (something happened)" 2) short answer, for now you should use "have you ever been to (or come) to Vietnam? Hope this helps!

1. Thanks/Thank you are correct. Thank is not. 2. Have been to Vietnam ... Have you gone to Vietnam. These two are both correct; we don't use 'in'. However, they have different meanings. 'been' means in your lifetime. 'Gone' means are you on a trip there now. Because of this, 'been' is the much more common one to use.