when your argument is based on him being a big need, it's kind of presumed that he has to make an early impact
otherwise, the "need" argument is irrelevant
You are oversimplifying it.
The Saints NEED a LT...and they drafted a guy who will most likely require a year before he is ready. Why? Presumably, it was because there wasn't someone staring them in the face that would have been able to step in and play at a high level this year anyway...so they took a guy that had higher upside. In other words, the draft wasn't going to solve that problem this year.
Safety could be the exact same thing. They see the need to replace both safeties and perhaps think that Vaccaro could be a guy that replaces one of them soon. But, if the kid takes a little while to figure it out (very common for a rookie), and it is next year before he is ready, it doesn't disprove that safety was a huge need.