It is definitely possible to add too much bleach when you have a vinyl liner--I would shock it to the 15 ppm that your CYA indicates. The only problem with covering it at shock level is that the fumes may fade your cover--is it possible to leave maybe a corner of the cover off in order to vent it? The other option might be to add some polyquat 60, let it circulate for a bit, then bring your chlorine back up to normal levels again before covering it. The worst thing that will happen is that you'll have some algae when you return, and we can always help you get that cleaned up!