A VIP doesn't normally change anything in the URI that the client requests, so in this case how does the client know to request something other than index.html (or one of the other browser defaults) if the VIP is not there?
While you could certainly use an iRule to change the requested URI (something along the lines of "if the URI is / or /index.html change it to /whateveryourdefaultis.html"), it would need to be sure and capture all iterations of the possible URI's that would need to be converted, as well as ignoring ones that do not need to go to the default page.
Denny