I would recommend to you a recent book by Lydia Liu, entitled
The Clash of Empires. It argues that the term
Yi which the Qing court and ministers like Lin Zexu used to refer to the British was incorrectly translated as 'barbarian' by the British based on it having that connotation in the Confucian classics. However, in the mid-Qing period the term
Yi had actually been redefined by the Manchu emperors to simply denote 'foreigner' or 'outsider'. Therefore the Yongzheng emperor freely recognized that the Manchus were Yi before their conquest of the Ming empire, but asserted that Yi-ness had nothing to do with culture and everything to do with geography.
QUOTE
Was any country that accepted Confucianism and was sinified (like Korea, VIetnam, and Japan) considered civilized?
It depends what definition you use for 'civilized', and what definition you use for 'sinified'. These concepts
per se did not exist in imperial China. Other countries could never be considered equal because they were not directly under the
hua (morally transformative power) of the Son of Heaven, but those who were sufficiently respectful and admiring of the Empire and emulated the Confucian rites and virtues were seen as being in the process of transformation and improvement. You could say that beyond the Empire there were 'morally developing countries' and 'morally undeveloped countries'. The 'morally developing' ones are the ones we commonly label as 'sinified' or 'civilized from the Chinese point of view'.
Korean and Vietnamese kings or ambassadors in the Ming-Qing period were occasionally annoyed at being referred to by the Chinese imperial court as Yi or Man. They believed such terms were only fit for their non-Confucian neighbours. However, it should be noted that this Confucianized hierarchy of states in East Asia only really took shape in Ming and Qing times. Before that, the Koreans and Viet do not seem to have been very interested in asserting cultural superiority over other peoples.