The big issue is more complex. I'd have no issue eating food which had had extra vitamin genes spliced in, and verified to be producing what its supposed to.

However much of GMO is of two types:
1) Resistant to poisons that kill weeds. The poisons can then also remain in the GMO crop. For example round up. While much lower toxicity compared to agent orange, its not something I want to be eating every day! Yes residue and metabolites end up in "round up ready" crops.

2) Insect resistant crops. Sounds great until you hear how it works. The plant is taught to produce its own insecticide. In some cases these are exotic RNA based insecticides. They are not tested for lack of effect in humans, and there is no reason to believe they should be free of effects.

Instead of worrying about GMO, we need to be worrying about the particular toxins applied to or created in GMO crops.