How to know if an keyboard/input related app is safe to use
You can't. A keyboard, by nature, is required to know everything you type, including your account credentials. Your best bet is to select keyboard coming from top developers.

My favorite? Multiling Keyboard, supports multiple languages (including Vietnamese), highly customizable, the only keyboard I know of that doesn't require Internet permission (so the keyboard can't send what you are typing on the fly).

Some other popular keyboard that support Vietnamese includes Swiftkey (free), Swype (paid).

As for Vietnamese products, Laban is fine (comes from Vinagame I think), Gotiengviet also was used by hundreds of thousands users without any incident.