/etc/X11/Xvnc-session is in tigervnc-standalone-server 1.7.0+dfsg-8ubuntu2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | #! /bin/sh
test x"$SHELL" = x"" && SHELL=/bin/bash
test x"$1" = x"" && set -- default
vncconfig -iconic &
$SHELL -l <<EOF
exec /etc/X11/Xsession "$@"
EOF
vncserver -kill $DISPLAY
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