Quoting from the article:

> Vietnamese government agents tried to plant spyware on the phones of members of Congress, American policy experts and U.S. journalists this year in a brazen campaign that underscores the rapid proliferation of state-of-the-art hacking tools, according to forensic examination of links posted to Twitter and documents uncovered by a consortium of news outlets that includes The Washington Post.

and a second report by amnesty international about hacking tools/targets: https://archive.ph/n9BQ0

so, its not likely that someone looked the source IP being from Vietnam and made the conclusion, it seems there is more analysis and evidence behind this article.