That's a valid point, but I feel there's a few things that need to be added for consideration. Vinnie, as his does his dad, thinks he's in a situation that is contingent on the system he's playing.
He didn't have the speed to keep up with those slot receivers when we were in nickel and he was playing the STAR position. He didn't like that position (part of the reason he's moving on) and he didn't play well in passing situations in that position.
However, if he's playing a true safety role? It is the opinion of the Sunseri's he can develop his skills to where he can man that position. They also look at him (if he's an undrafted free agent) as having the opportunity to make sure the team he "chooses" gives him a place to work towards that spot on the roster.
Is it good advice he's getting from his dad? Debatable. Sal does have his experience in the league. With no doubt it's advice given with an emotional component as well. I'm hesitant to use a stereotype of "Italian families sticking together" but it certainly fits.