Since no-one else who knows about it has posted, I'll offer this, though without much expertise.
1) Have you tried setting the "Unicode" setting in options?
2) I found that if I just included characters which have codes less than 256 (i.e. are members of the ASCII/ANSI set? examples included øèµã) they would appear normally with unicode NOT set (in VDU 23), but NOT WITH it set. However, as soon I added even one character which required a second (non-0) byte (I chose ῒ), ALL the "extra" characters flipped into unicode (which changed their identity), and needed to be shown with "unicode" set and with the VDU23....8
I initially thought you might need to choose a (different and) Unicode-compatible font (Look at Options/set font..., untick "use system fixed font", and choose a unicode font: the menu here offers me @Arial Unicode MS, which I deduce is a unicode-compatible monospace font,so it shouldn't break the things that rely on a monospaced font), but actually once the "unicode" option is selected, fixed system seems to work fine, so presumably it supports unicode.
So my conclusion, which may or may not be correct, is that at least some accented characters can have different codes in ANSI and in unicode, and you need to make sure you are using the right ones for the context.
Anyone who knows better (Richard is really good on this!), please correct/expand/clarify.