The other reason is that the old models of J10 and Su-27 were not designed to fit WS-10. It is either wise nor economic to modify these planes to the new engine.
You can see that there are at least 3 054A frigates and two 053 frigates in the background of the second pic. It is interesting to see that the Navy is changing the format of hull numbers, from traditional black style to a more US style.
The Navy don't need to know everything about Viet's 'new' submarines to find out a way to handle it. Plus it takes years before Vietnam receive all of the 6 ships which cost them $2 billion and more years for them to be familiar with the ships. Navy is getting new models to replace old ones...
Inflation... $2000 in the 70s is about $4400 today given a 2% inflation rate.
IJN has become a history. Today's Japan is nothing but US's puppy dog, a UK on the other side of the World Island...