Both are narrow 17" seats, but I'm afraid a set of three this wide will not fit. You should measure this your self, but the rear hip room on the Vibe is listed as 47.8 inches. If we divide that by three it means each seat has 15.9" of space.
How tall is your 7 year old? Can you get a narrow booster seat to fit them?
Disclaimer: I'm not a parent. I have no practical knowledge in this arena.
Edit: The numbers above where for the 03 GT. Looks like the 09 GT has less rear Hip room 43.7 in. That's going to be a tight squeeze. If both child seats are currently installed in the outboard locations have your 7 year old sit in the middle seat and try to put on the seat belt.
06 Vibe Base - Mono Platinum - Manual