Fans of this show, and especially fans of Milo Ventimiglia’s Jack, have been through a rough stretch lately, having to endure everything up to, and including, his death. But because This Is Us deals so liberally with generational flashbacks, tonight’s new episode, “Vegas, Baby,” is a lot lighter. In the more modern timeline, it presents the bachelor and bachelorette parties for Toby (Chris Sullivan) and Kate (Chrissy Metz). And in flashbacks, we go back to years before Jack’s death, when he, Rebecca (Mandy Moore) and the kids are planning Jack and Rebecca’s wedding anniversary – in conflictingly different ways.