When do you yangu and langu ?
Fulana is a U/N (9/10) noun.
fulana yangu singular.
fulana zangu plural.
langu is not used with this noun class.
Is it interchangeable?
No they are not interchangeable. Every possessive ( e.g. changu, langu, yangu, vyangu ...) has to agree with the noun class of the possessed noun.
This dictionary says fulana means "undershirt, jersey". Is there any version of English in which "vest" means this as well? By the way, searching for fulana images in Swahili I saw mostly t-shirts.