There are three main oil seals in a older Vespa engine:

• Main bearing oil seal (at the crank): Keeps the gearbox oil from mixing with the oil/gas mixture.
• Flywheel side oil seal (at the crank): Keeps the oil/gas mix from the outside of the casing.
• Rear wheel oil seal: Seals the gearbox oil around the rear axle.

There are also some more minor seals around the kickstart quadrant, the clutch cover, and the rear wheel back plate.

The main seals should be replaced every time the engine is taken apart as they are cheap to buy but they are buried pretty deep in the engine. If you have the engine apart you might as well drop them in.

Flywheel side seal: The flywheel oil seal on an older model large frame bike is different to the P series equivalent. The main difference is that the engine uses the same large bearings on both sides of the crank and both main bearings are lubricated by the gasoline/oil mix. On P series bikes the clutch side main bearing is actually lubricated by the transmission oil because the oil seal is located between the crank and the bearing...Anway...

The following images assume you have already dropped the engine and split the cases.

Once the casings are split you can flip over the flywheel side casing and use a large socket to remove the seal by tapping it through the casing. It doesn't matter if it gets damaged as it is going in the trash. 

In high mileage motors they can look pretty nasty. Clean the area all around where the main bearing and seal go. Be sure to have a look where the arrow is shown above to make sure that the bearing lubrication hole is clean. Blow through it with air if necessary. If this hole is blocked your bearings may not get proper lubrication.