I will be using a 70-300mm VR lens at a party in a couple of weeks. Unfortunately, I will not be able to use a lot of flash, so as the lens is only a f/4.5-5.6, I will have to use a slower shutter speed. I understand the limitations of VR in that while it will help me prevent camera shake, it will do nothing to help stop any motion in my subjects. Fortunately for me, I am only trying to shoot people at a party and not sports or fast action. Assuming people are just standing around, talking, etc, how slow can I go and still expect to capture sharp expressions, laughter, the occassional hair flip, etc? Obviously, an f/2.8 would be better, but I can't afford to buy it, don't know anyone with one, and there is no place to rent it where I live, so that will not be an option.