This VirtualBox bug was the reason that I started reading more about sendfile and encountered this article about nginx optimizations. The bug actually causes really weird behavior, in which extra bytes are added at the end of the file.
Do you have any links specifically discussing the issue?
I then found out that it is actually mentioned in the Vagrant docs also: https://docs.vagrantup.com/v2/synced-folders/virtualbox.html
Google for 'virtualbox sendfile' and there's a lot of discussion.
Worth noting, and this bit me recently, Go's http file handler also uses sendfile beneath the covers.