The question has multiple answers.
1. I think he has a better chance of being drafted high if he plays offense. He is a standout offensive player.
2. As far as how long he could play in the NFL, as was posted above, safeties have a much longer career. So, if long-term money is what he wants, he should play safety.
3. Everyone is talking about him playing running back. I think he could make an excellent slot receiver. I suspect you will see him playing slot (limited) in the weeks to come.
Regardless, Vaki is a stud. I am glad we have him.