Free international roaming is one of the many nice perks T-Mobile offers to entice customers. When the Uncarrier launched the feature over a year ago, it supported over 100 countries. That list has now grown to over 120. The latest addition includes Paraguay, in South America, and Croatia, in Europe.
T-Mobile provides international roaming to its Simple Choice customers, so don't use your phone all willy-nilly while globetrotting if you aren't one. And even if you are, don't expect the fastest speeds. They're not going to compare to what you get at home (unless, that is, your T-Mobile signal sucks). The idea here is to keep you connected, not to satisfy your on-demand video cravings.
For some reason, Vietnam has disappeared from the list of supported Asian countries. You can see for yourself at the link below.