I have had Monitor
, Pacific Plus, Aries
, and Navic vanes, I find they work
best when you can balance the boat to where she nearly sails
herself, then engage the vane.
Watching your video the wind (from the flag on the stern, appears to come from off the port quarter, closer to a run than a broad reach.
I do not think you are overcanvased at all. Your jib is a bit oversheeted, naturaly because if you let it out more it would collapse and flog around as the boat turned in a more downwind direction. But this also makes it want to head
up quick when she rolls to Stb or even heads up a little, thus deflecting the vane blade fully causing the helm
to come hard over, she heads off, boat slows, goes down wind vane corrects and the oscillations continue.
Always remember speed is stability (until you surf… )
My recommendations to correct this is to get that jib out on a pole so you can adjust it to the wind (ease it out)without it collapsing or being over sheeted.
When you took her by hand, it was probably easy to sail until the boat healed to stb and then you would have to give a good amount of rudder to catch her swing to port and bring her back, then as she started to go a bit much downwind a bit of ruder, wait a moment and she would come back. The vane does not wait, it just stays over until the wind rights the vane. Also when she heels to stb and starts to round up the apparent wind speed increases blowing the vane over, more downwind the apparent wind decreases having less effect on the vane so she goes too far.
I do not recall
on the Navik, can you adjust the tilt the blade a bit downwind? This decreases the sensitivity of the vane.
Anyway, balance the boat, play with the vane, and you will get her to steer. And she will steer better with a bit too much sail than with too little.
Hope this helps,