Stable or not, preview links only works for people who have edit access to the post.  So I doubt it will meet your needs.

Do you want them vendor to see the post in advance of your publishing it to the general public?   If so, you could publish the post in a private blog, which you have invited them to read - or just email it to them.

Or if you want to share details with them only on the same day as everyone else, but want to pre-warn them that your post is coming - then you could just use the custom setting in the Links option in the post-editor to set what the URL will be, ie

www.yourBlog.blogspot.com/2015/05/yourCustomURLtext.html