Can constipation cause one off vomiting in a 2 year old?

Yes: I don't know what you mean by "one off vomiting, " but i've seen constipation severe enough to cause vomiting in a child. This is, however, a rare thing, and the constipation has to be both very, very severe and very long standing. If your child is having constipation and vomiting, it's more likely that these are two related symptoms and less likely that one is causing the other.