If you haven't raised your upper fairing on your 2008 by installing the 2010 upper fairing brackets then the ONLY fairing lowers that will fit will be the NON-VENTED style fairing lowers that were made specifically for the RG.
You can still put vents in those. A number of members have done this. That's how I found about about them.
2005 FLTRI Black Cherry, 95" Big Bore, Thundermax w/Auto Tune, V&H Dresser Duals w/Klock Werks SS, Andrews 26 Cams, SE Stage 1 intake, SE OD/6 Speed, Wards FCS cooling fans, Krome Werks Stretch Heritage +2 bars, Progressive Monotubes, 444's, & Touring Link, Michelin Commander II's, 14.5" Madstad, Lowers with Classic Motorcycle Vents, detachable King Tour Pack, Daymakers