Yea, at 8psi a week drop you definitely have a leak. If you have a valve core tool I would try and tighten them. They can come loose. I fought with one on my Jeep for a while before replacing the whole stem. It would hold pressure fine for weeks, then suddenly go flat overnight. Tighten the valve core and repeat the process. Luckily I have an on board compressor! I've also battled the stem that leaks between it and the wheel. Usually blamed on mud/debri getting packing in while off roading. I doubt your Vibe has that issue though
