"She is forever mine."
Translation:Cô ấy vĩnh viễn là của tôi.
vĩnh viễn: more emphatic, used a lot in expressions where you expect never to see someone or something again.
mãi or mãi mãi: general word for forever
luôn or luôn luôn: general word for always
The difference between forever and always is that forever implies continuity (the universe might last forever) as opposed to (the universe might last always) while always is used for continuity within a specific environment (he always catches the bus) as opposed to (he forever catches the bus).