Not sure there's a difference. I've seen "mãi mãi" used more in less formal writings or in speaking, but I'm pretty sure that comes down to the preferences of the writers more than any formalized differences. Might also be because "vĩnh viễn" doesn't sound nearly as nice when spoken by us southerners. Yikes!
I was part of the original team to save the show, called "Foreverists", later "Forever Warriors." I helped spread the petition (Now with 100,000+ signatures) to return the show, and was supported by Matt Miller (The show's creator) and the cast via their encouragement on Twitter. Matt was sincerely grateful for my helping organise the events on Twitter to get Forever hashtags trending, and to try to save the show through the petition. However, we didn't manage, as much as we tried, and as much as I sincerely prayed over it. The reason being that Disney cancelled it, is that ABC is owned by Disney, and the show was on ABC, however Warner Brothers was the one making profits from it, as they owned it. But I feel like something else happened, as Warner Bros wouldn't even release the DVD's, we had to petiton THAT as well, and finally got them (With an online only release, unfortunately.) I'm sincerely glad to meet another Foreverist, and I just wanted you to know we really did try to save it. God bless! :) <3
"Vĩnh viễn" to "luôn luôn" is the same as "forever" to "always".