That is not supported at the moment.
There used to be a plugin for Photoshop that could do it, but I don't know if it is still available.
Wow, OK, that sucks. Thanks.
The only solutions I've found to save vector Illustrator graphics as WebP are the following;
1. You either copy paste the vector into a PSD and use WebPShop plugin (it's free), or
2. You place an Illustrator file in the Photoshop doc and use the plugin mentioned above.
Here's the link to the plugin: https://developers.google.com/speed/webp/docs/webpshop