How can i share story source file with Vendor

If you are intending for them to review it, or change it, they have to have StoryLine to do so. If you just want them to have it, it's just a computer file. If it is small enough, you can send it as an email attachment. Otherwise, if you have cloud storage (like dropbox, Google Drive), put it there, set the permissions appropriately, and send them the link. Or you could copy it to a CD, DVD, or thumb drive and send it to them.