In my experiences while flying with VN, there are actually 2 sides to VN;

1. Short Haul flights to SE Asia & Southern China/Hong Kong
2. Medium Haul to Long Haul flights to Australia, Europe, Japan, Sth Korea

All the flights in let's say 'Zone 1' that I have taken with them had been with nice, polite and friendly crew, who would even talk to you in a polite manner with exceptional service levels, with clean aircrafts, while their 'Zone 2' counterparts are very pretty much the opposite, no smiling, and very rude behavior to all pax even in J, although the aircraft varies.