If you need to call actual phone numbers (not just other app users), these services offer dedicated monthly plans for Vietnam:
Finding reliable ways to make often depends on whether you are calling a landline or a mobile number, and if you prefer using a data-based app or a traditional phone service. 1. Top Unlimited Calling Plans (App-to-Phone) Unlimited Calls To Vietnam
: Offers unlimited calling to both Vietnamese landlines and mobile phones for $25.99/month. If you need to call actual phone numbers
For 100% free unlimited calling, both parties must have the same app installed and a stable internet connection: Unlimited calling to Vietnam from USA using Ooma Telo Unlimited Calls To Vietnam