Fist off, welcome to EM!
Have you looked through our wiki of mods?
Car MPG Efficiency Modifications Main - EcoModder
I do not have a vibe but I have its better half a Toyota Matrix (same car different trim).
As for what mods have been done I think I have done the most to a Matrix/ vibe:
Grill block
smooth hub caps
Kamback
Boat tail
bellpan
Mirror delete
and more
I took a manual matrix from 31 MPG to 52-53
