Using English iInternational I can type "`" in Notepad and "a" in UniTextBox and I get the combined "à" in UniTextBox.
Same for keyboard ABNT2 Portuguese, type "~" in Notepad and "a" in UniTextBox and I get "â" in UniTextBox.
In both cases it works the other way around, accent in UniTextBox and "a" in Notepad gives "á" or "ã".

Tanner_H
Vietnamese:
In UniTextBox I get the correct character with 1, 1+9, 1+8
ă
ặ
ắ

In ComCtldemo I also get:
ă
ặ
ắ

wqweto I've tested CyberActiveX's Unicode rich-textbox (the free download) and it is not working with my keyboard (I'm a native Bulgarian using cyrillic alphabet with a twist
There are 2 RichEdit controls that we distribute:
One is UniRichEdit_BDC0849A.ocx, a wrapper for vbRichTextBox and we would not expect it to handle IME correctly.
The full Unicode version is UniRichEdit100_EDA1811C.ocx. This is the one that handles IME using same technique as UniTextBox.
You need to download UniSuitePlus to get this control.
UniTextBox is part of UniSuitePlus_BDC0849A.ocx which has 27 controls as a single ocx.