Why are some keyboard characters unavailable in Virtual Mode?

The cause may be that the relevant keysym is not available in the X keymap table (xmodmap -pke). If you cannot enter a particular character, try closing the application, restarting it, and entering the same character again; it should now appear since the act of entering it the first time dynamically adds the keysym to the table.

For a more robust workaround, you could try explicitly adding the keyboard in use on the virtual desktop to the xstartup file, and restarting RealVNC Server.

For example, adding the below line near the top of /etc/vnc/xstartup.custom or $HOME/.vnc/xstartup, before any commands to run applications:

setxkbmap -symbols "pc+us+us(intl):2" -print | sed -n '/xkb_keymap {/,$p' | xkbcomp - $DISPLAY

adds US and US international keyboards to RealVNC Server.

Alternatively:

setxkbmap -symbols "pc+cz+ru:2+gr:3+fr:4" -print | sed -n '/xkb_keymap {/,$p' | xkbcomp - $DISPLAY

…adds Czech, Russian, Greek, and French keyboards.