You're welcomeQ1: A little bird said if I were a man, I couldn't greet a pretty woman by saying, "Hey, Good-looking'.
Is it true? Why?
Yes, you generally wouldn't do this unless you knew the woman well, and knew she wouldn't mind it. Otherwise it could be seen as sexual harrassment, or at least sexism. Of course, the situation makes a difference. At a night-club it would be acceptable, but it's a pretty lame pick-up line.
Q2: Is it vulgar to say "I laugh my ass off"? Some people suggested it was bad to say that and I had better say " I laugh my head off", but some people thought it was okay. I am lost. What do you think about?
Yes, it is vulgar, generally. Don't be fooled by what people write on the internet. Real life imposes stricter standards.
If some people say it's OK and some people say it's vulgar, then you don't use it unless you know the people and what they generally think.
Thanks in advance!