Here's a thread where they measure it.
VBB measurement help
Can you post a photo of your Vespa? You may not need the whole legshield.
That part is high quality, but it requires a lot of modification.
- There are two or three brace struts that need to be added
- The horncast needs to be welded in
- There is a reinforcement plate that needs to be fabricated and spot welded
It's possible you can use just the lower part of the legshield, though.
They make those so they can be used on a number of models, but the model-specific parts need to be added.