Vietnam Coin ID And Grade Needed

Not so much a "counterfeit" as a "fantasy" - it's a charm, resembling a coin but not intended to be used as one. Large, "coin-like" charms such as this are popular in East Asia, especially in Vietnam where they are traditionally nailed to houses. They can always be found at the "tourist trap" street markets.

Annamese rebel king Quang Trung did not issue official "large" cash coins for circulation; a few large Quang Trung "coins" that are unquestionably old do exist, but their exact purpose is unknown; they are usually labelled as "presentation pieces" in the reference books. I don't think the genuine presentation pieces have this good-luck inscription on the other side, but I haven't been able to find a pic of a genuine one to be sure.

Your charm is a match for the inscription on the third charm down on this page, though the reverse characters on yours are written in a somewhat less tidy script.