Actually for the most part it doesn't. If it did, you'd pick a quarterback, because if he turns out good he'd be worth $20+ million and you'd be paying him $3 million. It would give you, by far, the best cost over replacement.
But of course if you draft a guy who isn't any good or that your team doesn't need simply because of what value he may provide over replacement before too long you are going to be drafting guys whose rookie contract averages well over $3 million per year.
I mean sure, if you think that one of the running backs and one of the offensive linemen are both likely to provide you with the same value then you can/should take replacement cost into account. But if not, you have to pick the guy that you think is more likely to be better.
I never said to just draft anyone regardless of position based on VOR. Obviously they shouldn't take a guy rated at 50th overall because he's a QB.
What I said was the Steelers have a lot of needs and 3 of those - pass rush, CB, OT - are premium positions that represent massive savings compared to what you can get on the market. Consideration should therefore be given to the maximum potential return of taking an OT that might play for 10 years versus an RB that might play for 5 (and who is more replaceable on the market). I would just (almost) never burn a 1st on a RB unless I was a team, like KC last year, that had very few needs. And even then I'm not sure it's a great idea.