Replace your ~/.vnc/xstartup file with this:

#!/bin/sh
# ~/.vnc/xstartup - start an XFCE session for TigerVNC
# load user X resources (if present)
[ -r "$HOME/.Xresources" ] && xrdb "$HOME/.Xresources"
# avoid GNOME/X session conflicts
unset SESSION_MANAGER
unset DBUS_SESSION_BUS_ADDRESS
# set runtime dir so pulseaudio and dbus work in user session
export XDG_RUNTIME_DIR="/run/user/$(id -u)"
[ -d "$XDG_RUNTIME_DIR" ] || mkdir -p "$XDG_RUNTIME_DIR" && chmod 700 "$XDG_RUNTIME_DIR"
# start a dbus session and then start XFCE
exec dbus-launch --exit-with-session startxfce4