Can my child still get the rotovirus even if vaccinated last winter (oral vax). both of my twins (18mth) have diarrhea, but otherwise are ok.?

Much less likely: Three out of four children who had rotavirus vaccine won't have any symptoms at all if they come in contact with the infection; more importantly, almost no immunized child will become significantly ill with it. Keep in mind that rotavirus is only one cause of infectious diarrhea, so perhaps your twins have diarrhea from another cause. I hope they get better quickly!