As far as the vitamins, my niece ate Flinstones vitamins every day of her pregnancy, it may not be a pre-natal but it's better than nothing. They even have chewy ones that taste like gummy bears. Again, it's better than nothing and a LOT easier on the stomach.
With my 4th baby I lost weigh the first half of the pregnancy b/c I was so nauseated all day every day. It was all I could do to eat a little applesauce or a string cheese. Well, I assure you I made up for it the second half!!
I wouldn't worry too much as long as the baby seems to be developing ok!