FYI,
First, you do NOT want to flush your transmission! No long term good will come from such activity, just use your Google search and begin reading the delayed effects of flushing procedures.
Secondly, if you do anything, you want to exchange your oil in the transmission, a procedure that removes the old oil as it puts in new oil, a transfusion if you will.
I prefer the do it yourself technique of simply dropping the oil by pulling the transmission drain plug three times with short mileage intervals between each drop. Statistically this exchanges almost all the old oil in the unit. I now also pull the trans plug at every 2nd or 3rd oil change (14,000-20,000 miles +/-). The act of frequently Adding fresh oils with fresh additives gives me peace of mind.
At 50,000 my oil was filthy, they don't recommend a change until 100,000 but I ignored that and erred on the side of too often. I've now got 110,000 miles and all is well with the transaxle.
Any Toyota Dealer that won't work on your Pontiac is simply short sighted and turning down easy cash, the Vibe IS a Toyota Matrix / Corolla in all it's major components.
Personally I shy away from other oils that aren't Toyota T-IV but others use them without harm. Whatever you use it MUST be T-IV compatible!
Dave