Is it true that 30% of Buddhists in Vietnam belong to the Theravada school?

Nominally, yes. But actually very few Vietnamese are truly "Buddhists" the way Thais, Laotians, Cambodians and Burmese are. The Vietnamese, like the Chinese, practice ancestor worship alongside Buddhism. They may go to a temple /pagoda once or twice a year if that...

Some Vietnamese go to a temple/pagoda only a few times in their lives, mainly for funerals or memorial services for their dead relatives or friends. But any Vietnamese who's not a Christian would tell you he's a "Buddhist".

In the 11th century, the Vietnamese Emperor made Buddhism the official religion of the realm. Buddhist temples/pagodas were built in just about every village and town. So every Vietnamese, by virtue of being a subject of the empire, has to proclaim himself a "Buddhist" even if he did not really practice the religion. That tradition and continues into this day, except for those who have embraced Christianity ( or who belong to ethnic minorities who are Muslim or animists.)