In May PD updated with content twice 2 weeks apart. We don't know that September is even a big update, could be just the VGT like when the BMW was added. Plus PD has been saying they want to keep updates monthly so it's still very very likely there is an August update.
EDIT: And there still isn't any actual business sense in putting the Toyota VGT with Zahara because business wise all the media is done with Toyota VGT because everything they need to know about is available at unveil, they don't actually care when it makes the game.
Trust me, September's update(s) will be big, PD has to be holding some stuff back for the next few months. And it stills makes business sense to add the Toyota VGT in with Zahara as the some media outlets report when the VGT's hit the game. So you're wrong there. As for PD wanting to keep updates monthly, if that's how you're basing your prediction on, I'm sorry to say this but there has already been an update this month so PD has already checked off their monthly update off.