Can I use my Samsung Galaxy S5 overseas in Vietnam?

I'll be traveling to Vietnam from the US in a few weeks. Can I switch my SIM card to one purchased in Vietnam while I am in country, or are there any complications that could arise from this?. I'm currently with Verizon and don't want to pay for an international SIM card. I should mainly be using messaging, data (for whatsapp) and wifi for updating my family while I'm away. On a side note, I do have an old iPhone 5C that could possibly be used if it would work better.

Could anyone provide any advice for this?

Are there any other options I could consider?

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