AFAIK no bandwidth monitoring tool is better than vnstat. Unfortunately it does't support this feature. But you can achieve this by using multiple databases for vnstat.

Save the data for on peak hours to vnstat1 and for off-peak to vnstat2. Make a daemon script that will switch them based on time of the day using an if condition in an infinite loop; if not between 02-08 hours use vnstat1 else disable vnstat1, enable vnstat2 and sleep.

I wrote the following bash script. Save it in a file and make it executable using sudo chmod x <filename>. Test it for some time. Finally make the script auto start with OS. Run it as the root user. (Just add the path to the script in /etc/rc.local for it to be executed at boot time).

#!/bin/bash

# vnstat_switcher.sh



# Select the interface to monitor e.g: eth0, eth1, ppp0

i=eth0



# Location of database used by vnstat

db1='/var/lib/vnstat1'   # on-peak

db2='/var/lib/vnstat2'   # off-peak



onpeakdb='/tmp/onpeak'

offpeakdb='/tmp/offpeak'



debug=false

iscatdata=true

current=0



# Create database for db1 if it doesn't exist

if ! [ -d "$db1" ]

    then

    mkdir -p "$db1"

    vnstat -i $i --dbdir "$db1" -u

fi



# Create database for db2 if it doesn't exist

if ! [ -d "$db2" ]

    then

    mkdir -p "$db2"

    vnstat -i $i --dbdir "$db2" -u

fi



$debug && echo 1

#vnstat -i $i --disable



while true

    do

    t=$( date  %H )

    $debug && t=$( date  %S )

    if [ "$t" -lt 2 ] || [ "$t" -gt 7 ]    # if peak hours

        then

        $debug && echo 'On-peak hours'

        vnstat -i $i --dbdir "$db1" -u

        $debug && echo 2

        if [ "$iscatdata" ]

            then

            vnstat -i $i --dbdir "$db2" > "$offpeakdb"

            iscatdata=false

        fi

        vnstat -i $i --dbdir "$db1" > "$onpeakdb"

        if [ $current != 1 ]

            then

            vnstat -i $i --disable --dbdir "$db2"

            $debug && echo 3

            vnstat -i $i --enable --dbdir "$db1" --sync

            $debug && echo 4

            current=1

        fi

    else

        $debug &&  echo 'Off-peak hours'

        vnstat -i $i --dbdir "$db2" -u

        $debug && echo 5

        if [ "$iscatdata" ]

            then

            vnstat -i $i --dbdir "$db1" > "$onpeakdb"

            iscatdata=false

        fi

        vnstat -i $i --dbdir "$db2" > "$offpeakdb"

        if [ $current != 2 ]

            then

            vnstat -i $i --disable --dbdir "$db1"

            $debug && echo 6

            vnstat -i $i --enable --dbdir "$db2" --sync

            $debug && echo 7

            current=2

        fi

    fi

    $debug && sleep 1 || sleep 1m

    done





#### Notes

# Ignore this message at first execution

# Interface "lo" is already disabled.

# Interface "lo" is already enabled.



#For debugging use interface 'lo' then

# `ping -s 2222 0.0.0.0`

Change i=eth0 on 5th line to the network interface you want to monitor.

To know the usage run cat /tmp/onpeak and cat /tmp/offpeak respectively.

Also it's a good idea to reset the data on the default database (/var/lib/vnstat), as it will never update if everything in the script goes well.