/usr/share/munin/plugins/vlan_ is in munin-node 1.4.6-3ubuntu3.
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 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 | #!/bin/sh
#
# Wildcard-script to monitor network interfaces. To monitor an
# interface, link vlan_<interface> to this file. E.g.
#
#    ln /usr/share/munin/node/plugins-contrib/vlan_ /etc/munin/node.d/vlan_eth0_1
#
# ...will monitor eth0.1
#
# Parameters:
#
# 	config
# 	autoconf
# 	suggest
#
#%# family=manual
#%# capabilities=autoconf suggest
INTERFACE=$(basename $0 | sed 's/^vlan_//g' | tr '_' '.')
if [ "$1" = "autoconf" ]; then
    if [ ! -d /proc/net/vlan ] ; then 
	echo "no (no vlan support)"
	exit 0
    fi
    if [ ! -d -r /proc/net/vlan ] ; then
	echo "no (cannot not read /proc/net/vlan)"
	exit 0
    fi
    if [ ! -x /proc/net/vlan ] ; then
	echo "no (cannot cd to /proc/net/vlan)"
	exit 0
    fi
    echo yes
    exit 0
fi
if [ "$1" = "suggest" ]; then
    for file in /proc/net/vlan/*; do
	if [ ! "$file" = "/proc/net/vlan/config" ]; then
	    basename $file | tr '.' '_'
	fi
    done
    exit 0
fi
if [ "$1" = "config" ]; then
	echo "graph_order received transmitted" 
	echo "graph_title VLAN $INTERFACE Traffic"
	echo 'graph_args --base 1024'
	echo 'graph_vlabel bits per ${graph_period} in (-) / out (+)'
	echo 'graph_category network'
	echo 'received.label bps'
        echo 'received.type DERIVE'
        echo 'received.min 0'
	echo 'received.graph no'
	echo 'received.cdef received,8,*'
	echo 'received.draw AREA'
        echo 'transmitted.label bps'
	echo 'transmitted.type DERIVE'
	echo 'transmitted.min 0'
	echo 'transmitted.negative received'
	echo 'transmitted.cdef transmitted,8,*'
	echo 'transmitted.draw AREA'
	exit 0
fi
awk "/bytes received/ { print \"received.value \" \$4 } /bytes transmitted/ { print \"transmitted.value \" \$4 }" < /proc/net/vlan/$INTERFACE
 |