I have a 2006 Pontiac G6 with 160k on the 3.5 motor. Around 155k the brake pedal went hard and spongy. Also, about 1 in every 40-50 times I start it cold, the starter will turn without a successful start. After this occurs, it starts immediately without a problem. The starter motor was replaced about 36 months ago at 140k. I'm suspicious it's a vacuum leak, but it doesn't hiss at idle. Could it all be a vacuum leak or could I have a bad starter as well?
I think you have two completely separate issues here (brakes & starting). Since the engine is turning over, the starter is not bad. If there was a vacuum leak causing unmetered air to enter the engine, it would be running differently. It could just be that your MAF or other intake located sensors need cleaning for the starting.
As for your brakes, it would seem that your vacuum assist canister is not getting the vacuum it needs to help with the brakes. I'd check to ensure the valve for the vacuum line has not been damaged or is at least in good repair. If that isn't it, you could have a leak in the bellows which does the assisting within the canister.
Sometimes small vacuum leaks can be partially compensated by modern engine systems.
An old trick is to spray carb cleaner, water, or anything else that will affect idle around all the suspect hoses. If the engine surges or stumbles when you spray, you have a vacuum leak. Try more than one type of spray one suppressing (like water), one boosting (any flammable volatile organic).
The brake vacuum booster (assuming you have one) can hide a vacuum leak inside, or even feed the engine brake fluid in extreme cases. Remove the booster vacuum hose, let it run open and plugged, and see if that changes engine operation.