Music stores in the U.S. usually do not have melodicas in stock, kyri, and is highly unlikely to find an M-37C in stock here. As far as I know (and I follow the market), the M-37C always ships from Japan.
I have a friend in Vietnam to whom I have sent packages. The shipping is expensive from here. To have an M-37C shipped here from Japan is about $40-45. Do you think it would be more than that to Vietnam? You can check for the M-37C on Rakuten, Buyee, or Chuya and see the shipping cost. Would you have to pay import duty on a single melodica in Vietnam? That doesn’t sound right. If so, you could avoid that by having it sent as a gift, which I guess would involve getting it ordered in someone else’s name.
But you might also check Amazon.com Japan and see what they say about shipping to Vietnam.