is there any reason why "whilst" should not work here?
No. Use the report feature so the mods can add additional translations, they can't always think of all possible translations in advance.
What about he sleeptalks- that's a legit term in my country similar to sleepwalking
How would the general meaning change if this was just "Anh ấy nói khi ngủ." - He talks when sleeping. ?
Sounds weird. Shouldn't it be, anh ay noi truyen trong khi ngu
"He talks in his sleep." This should be an acceptable translation.
It is now, that's what I answered.
Now that's what I call talent!
He talks while asleep