[Question] Traveling back to the US (non-citizens)

Ok I'm a US citizen and I'm trying to plan a vacation this xmas/new yr to Vietnam and Hong Kong with my relatives, but the problem is that my uncle arn't US citizen (here on green card from Vietnam).

I know they can go to Vietnam with no problem since that's their homeland and need to get a visa to Hong Kong but the question is (I'm getting different answers from airline agencies, goverment website) what do they need to RE-ENTER the US without any problems?

I was told they need a Re-entry form? then I was told you dont need one if you're gone for less than a year (gov. website), then was told you do need one just in case but it would take 5-6months to apply for one?

Well I'm kinda in a hurry to buy the plane tickets and plan out everything now so was wondering if any of you guys might know these immigrations stuff better.

Just dont want them to leave on vacation but can't come back it for some reason if we dont have the correct paper work

thanks