The only seal between the magneto and engine is to stop oil escaping externally at that joint. There is nothing to prevent oil entering the magneto although the gasket paper flinger does help. The reason that oil enters the magneto is due to engine crankcase pressure and of all Ariels the VB is the worst because it displaces the biggest volume into the crankcase.
The best way to reduce crankcase pressure is to fit a one way PCV valve (reed valve type) to the end of the breather. That way the breather only expels air and does not suck it back in as the piston rises, but rather creates a negative crankcase pressure. Ideally the HT HS crankshaft oil seal should also be fitted to better seal the crankcase.
Today I had a thoroughly enjoyable ride of 200 miles at speeds between 60 and 70 mph two up. This is a couple of photos of my bike that I have just taken. There is no oil mist around the timing covers or chaincases. (but it appears that I have just developed an oil leak from the pushrod cups.)
This is a photo of my sons NH after his last ride of about 100miles at lower speeds. Note the oil mist around the timing covers and crankcase chaincase. It is crankcase pressure that forces the oily mist through the timing cover, chaincase gaskets and it is doing exactly the same into the magneto. With the OHV singles it is a matter of fitting a reed type PCV valve to the end of the timing cover breather and then check if you have any oily joints after a hard ride. If not then you can be pretty sure that your magneto is going to benefit.
The VB has a second breather off the tappet compartment but it is fed through a very small hole into that chamber. It would be necessary to fit a second PCV valve to that breather also. Incidentally my VB has not yet been fitted with PCV valves and has not exceeded 50mph on an engine that is not quite run in and yet I am amazed at how much oil mist is around the timing cover gaskets. You should remove the ball from the timing cover breather if you fit a reed valve to the end of the breather.