Well, there's a whole host of info that could help troubleshoot this question. Is the culture from grains or a non-propogatable commercial culture? Do you alternate or sanitize the vessels when transfering grains to fresh substrate? There are many types of kefir grains, most of which have a certain degree of acetobacteria (bacteria that either convert alcohol into acetic acid or the substrate directly). Keep in mind that kefir is a symbiotic culture of many bacteria. Therefore, as much of a pain as it is, the culture itself needs to be directed sometimes to reach the right proportions. It sounds as if the acetobacter is a bit too prevalent if it does smell of vinegar (instead of a slight souring). After your initial 24 hours, you might consider refridgerating for another 24 hours to build up the lactic acid producing bacteria that function at that temperature. Also, to round things out, you may consider adding yeast nutrient (availible at homebrew stores ie fermax) sparingly to even out any yeast elements of the culture that are lacking.