Vietnam is not modified from "CallerID-Format Fix iOS 5.x" because the 5.x standard version seems to work , so you can uninstall it.
But if you have installed is because you must have some problem.
Are you sure that Vietnam is set in Settings->General->International->Region Format->
Only if you use a SIM-adapter for a carrier-locked iPhone try this extra fix
for CPActiveCountryCode = vn (Vietnam)
save original file (if present):
substitute with (or copy) :
check file permissions = 644 (owner:rw- group:r-- world:r--) should already be