You can send to Notification Centre using Applescript (osascript )in you shell script.

There is a very good thread here that explains the various ways of either running only Applescript as a shell script or being able to mix applescript in with bash, for example.

I created a shell script with:

#!/bin/bash
theDate=`date '+DATE: %m/%d/%y TIME:%H:%M:%S'`

osascript -e "display notification \"$theDate\" with title \"test 1\" subtitle \" sub 1\""

I then created a LaunchDaemon in the same way you have and loaded it without sudo.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>notifTest</string>
    <key>ProgramArguments</key>
    <array>
        <string>/Users/userName/Documents/notify.sh</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
    <key>StartInterval</key>
    <integer>90</integer>
</dict>
</plist>

Every 90 seconds I get a notification:

The notification icon you will notice is the Script Editors one. Which means the Notification is controlled by the Script Editors Notification settings in Notification Centre

For those who don't want to read through the comments, it was a pretty simple fix after Mark gave me a bit of insight.

Initially, I wrote the vmstatus_startup script to call VM.sh which would then continually refresh the perl script (every 5 seconds). I was attempting to call the shell script from launchctl which just adds layers of complexity.

Instead, calling the perl script directly from the .plist and giving the keep alive key allowed it to run constantly after boot.

Finally, I had to make sure to add the perl directory to the .plist above the location of the script since launchd doens't have a PATH the same way a user does.