i sent The Vault-Tec Rep to covenant now can't find him anyone no how to get him back please help

When I sent him to Red Rocket it took him several in-game days to show up. Not sure how much time has passed in-game for you but it might be that.

It'll take him a while, if he didn't die along the way. Personally, I found him a real pain as when I took him to Sanctuary, his dialogue shrank to a simple "You know what? Since we've talked, I've been feeling swell! Look at us, here at Sanctuary! 'Prepared for the future!'" over and over again ad nausem.