iservices in VietNam

I am Vietnamese. I bought an iPhone 6 plus in Thailand 6 months ago. Now I have problem with my screen phone. I brought my phone to Iservices in HoChiMinh city, Vietnam. But they didn't accept to give my phone warranty. The reason they gave me is that my iPhone was bought in foreign country. Then they said that they will repair my phone with some charges and I must pay $350 USD to replace my screen phone. I can't afford it. Please help me to find better way.

ps: Do the iservices above have legal right to reject my request?