Actor Allusion: Towards Avatar, Angel and various other things other people did.
The Vegas episode combines Boreanaz's role of Angel (who also spent time in Vegas) with a role Angel himself played as vampire stuck in the fifties, rat-pack style. Booth spikes his hair up, dons greaser clothes and suspenders, faking a Bronx accent outta Guys and Dolls.
Booth, like Angel before him, feels that he has to atone for the lives he's taken as a sniper.
Simon Lee before being a mole in the CIA for Sophia, was training to be an astronaut. Probably to get him back to his home planet. (YMMV on whether Ian Anthony Dale is currently big enough now to warrant this.)