vncserver script starts new desktop based on WINDOWMANAGER variable. If none is set, it used to start xterm in twm. As we no longer distribute twm, switch to icewm.
Index: tigervnc-1.6.0/unix/vncserver
===================================================================
--- tigervnc-1.6.0.orig/unix/vncserver
+++ tigervnc-1.6.0/unix/vncserver
@@ -81,8 +81,8 @@ $defaultXStartup
        "[ -r \$HOME/.Xresources ] && xrdb \$HOME/.Xresources\n".
        "xsetroot -solid grey\n".
        "xterm -geometry 80x24+10+10 -ls -title \"\$VNCDESKTOP Desktop\" &\n".
-       "if [ -x /usr/bin/twm ]; then\n".
-       "  /usr/bin/twm &\n".
+       "if [ -x /usr/bin/icewm ]; then\n".
+       "  /usr/bin/icewm &\n".
        "else\n".
        "  echo \"No window manager found. You should install a window manager to get properly working VNC session.\"\n".
        "fi\n");