if your going on vacation for a couple of weeks or so, what i do is take the kefir grains and put them in the fridge with fresh milk, this will slow them down so they dont eat the lactose as fast, then when you come home strain the grains, throw away the milk (when storing you dont have to use a lot of milk,as to not waste it) and put them in fresh milk and they will begin like normal. However.....if your gone for like a couple of months they will have to be rejuvenated. To rejuvenate them you will have to put them in fresh milk, let it sit for a day, strain them, throw away the kefir (not the grains) and do the process again a couple of times till it starts making good kefir. Because what will happen is after they are in the fridge for so long they wont make quality kefir for a couple of cycles, after they have gone through a couple of cycles they will make good kefir again.
I personally like milk kefir better than water kefir, I dont digest well so its hard for me to get adequate protein and milk kefir is predigested protein plus other minerals and fat that you dont get with water kefir