Modern Vespa

I have no idea how to sort this wiring out. Aftermarket wiring harness makes a hard job of understanding what goes where when it comes to diagrams... can anyone please help me out

Welcome.

We'll get it figured out...
First, let's figure out exactly what ya actually have. US market 200, Euro market 200?

https://www.scooterhelp.com/electrics/pages/VSX1T.p200.html

Then find the right diagram above and work backwards following the wires from the stator up to the horn junction and headset and figure out what is what.

Grab a low voltage continuity tester for a few bucks... this will help and is easier to use solo than a standard multi-meter IMHO for this application.
I know... seems daunting, and I am a hack when it comes to electrical stuff, but the basic stuff is easy to sort with a little help.

I'm going to guess it's a US market P. So the wire harness could be either one of these two diagrams...

https://scooterhelp.com/electrics/wiring/VSX1T.USA.before.pdf
https://scooterhelp.com/electrics/wiring/VSX1T.USA.after.pdf

An easy way to find out which harness you have is by looking at which rectifier you have (either a 4 pole or 5 pole, which is located behind the battery).

Dgm16773om
Vsx1t
Is this of any use

Please see attached.... hope this helps

Does your P have a battery?

No battery

That Grey wire has been that way since I got
Have no idea why this is our where it should go

Does everything work? Could you just have spare wires if its an aftermarket loom?

Everything works but left rear turn signal....
The scooter engine will not turn off unless I stall it

Can you post a pic of the Stator or at least the wires coming off the stator at the junction where the harness attaches?

I wonder if this scooter has been converted over to have no battery? It's pretty common to do that conversion. A 3 wire AC regulator is what's normally used.

It does have only three wires.... no battery has to have been converted

Lucky man, batteries are such a pain in the arse and no bike of mine has one any longer. Sounds like you mainly need to fix only these TWO things and you'll be OK for now? I'm no expert on original wiring anyway, but I have a few ideas here from the wiring in my '78 P200 E. (Lets see a pic of yours!)

1.) If your left front turn signal works, it's likely that clever electrical contact concealed inside the tip of your cowl pin that gets the juice back to the cowl mounted light. Could be messed up at that cowl pin probe or its socket in the frame. You'd have to pull the tank to get at it - ugh, 1st time can be a small challenge if you have autolube, but you can just check for voltage in the cowl pin socket with a probe. Also check cowl wiring by grounding it and touching 12V to tip of cowl pin and bulb should light up.

2.) You kill the motor by your ignition switch grounding out the CDI unit, usually a green wire going forward to switch. The original green usually rots out for some reason, and is often replaced. I think you have power & lighting going to your switch but no kill wire. Maybe that stray red was a replacement and got disconnected? I wish I was in Greece and I could just go look...