Can you safely combine 5-http and vyvanse (lisdexamfetamine)? what are the benefits and risks?

Not yet: There is no reported drug drug interaction, however the national institutes of health recommends not to use 5-htp until more is known. 5-htp might be unsafe. Some people who have taken it have come down with eosinophilia-myalgia syndrome (ems), a serious condition involving extreme muscle tenderness (myalgia) and blood abnormalities (eosinophilia).

Yes, but why?: 5htp is precursor Amino Acid used by body in many areas. It is a precursor to Dopamine production in brain. But eating more of it won't increase brain dopamine. Dopamine is produced according to your genetic make up & putting in more won't change your genome or rate of production. A good analogy is over filling your gas tank (eating 5htp) hoping to increase your car's horsepower (vyvanse).