Hi Huongvan,
Welcome to this forum.
I'm not an English native too and have a 10 months old baby. Me and my hubby speak persian at home, but as I live in Malaysia I talk English to her when we are out! And we go to shichida class once a week where everyone speak English there.
I think you can show English and Vietnamese flashcards at the same time. I know some mothers who learn 5-6 different languages to their below 3 yo child and no mixing happened!
You can first start with Vietnamese flashcards and then show them in English. Just remember not mixing them at the same time. I mean show all cards you want to show in Vietnamese, after finishing tell her/him now we start English mommy and show the same cards (or different if you prefer) in English!
I hope this help you, good luck mommy