I know that you posted this quite a while ago, but here goes
1) send for your mother's death certificate (located either at county or state of death)- info could be important 2) send for your mother's birth certificate
These two documents may help you to find your maternal grandparents. You may want to do this quickly, they are getting older by the moment.Don't stop if they are deceased, your mother's siblings, aunts or uncles could provide medical info.
Medical inheritance comes from both your parents, so you need to find your father too. Your maternal side may know who he is or it might be entered into the family bible.
3) Have you gotten your original (pre-adoption) birth certificate?It might contain your father's name.If is does not, he may not know of you, which will make your search harder.
Was he a boyfriend or a rapist. If he was a rapist, there could be a police report filed on your mother's behalf.
If a boyfriend, he may have been in school with your mother - high school or college (don't forget instructors). Your could check her high school's year books (see her graduation year and up to two years earlier)looking for a senior boy that looks like you. It is possible that he went to Vietnam.