This one has be over abused in my work place by many until our boss finally put his foot down and (within his rights) said anyone sick during vacation time has to have his doctor's excuse (professionally) translated into German. Also not all excuses will be excepted, for example if you're bed ridden then your vacation time can be taken again at a later date but if you broke your arm that wouldn't stop you from enjoying your vacation time so you couldn't convert the vacation time to sick leave.