I think placing it after the crawl dilutes it, the crawl already tells you that Vader is obsessively looking for Luke, putting it after the crawl just seems redundant, especially as he is learning his identity here and the crawl implies that he knows it already.
I did a pre-crawl scene in my TFA edit, I think that it is something that can work very well if one doesn’t have inflexible ideas about how a Star Wars movie has to start.