The _vti stuff is there so the application (Front Page) can keep an eye on what you are doing. It keeps track of included pages, shared borders, stuff like that. It also uses it to keep a track of what has been updated so when you next publish it can decide ahead of time what to check for on your server.
You can delete them, but next time you open FP it will create them again.
If you’re not usnig FP, then delete happily.
(Anything that you do based on my suggestion is at your own risk).