The single line box at the bottom of mIRC window supports input and display of Unicode characters such as Vietnamese, but the channel window, where the conversations take place, cannot display them, showing ? instead.

you should use the search function before posting a bug to see if this wasn't addressed already... if you would check a few posts below your you would see the latest addition to the long ongoing debate about unicode

Unicode support seams to be broken in mIRC 6.03+

If you need unicode support use 5.91.

Actually, it hasn't been implemented yet... You seem to be confusing Unicode with one of the other Cyrillic encodings.

mIRC didn't have Unicode support in 5.91.

Oay let me re-phrase that....

Non-windows-characters (hebrew, cyrillic etc) work fine in mIRC 5.91 and seam to be broken in mIRC 6.03.

Unicode is not supported as Rounin says.