That's true but the issue here is not so much the type of craft but
Antiship missile vs torpedo , and which one is a better tool to sink another warship
Does the torpedo offer any advantages over the missile in this period of 70s /80s ?
Nope. Missiles all the way.
So Missile offers longer reach? but can the torpedo boat adopt any specific tactics to close the range with their opponents
Can MTB close the range in certain circumstances without being picked up by radar ?
Or can they shadow larger ships so it's hard to distinguish them from it by radar ?
There are very specific circumstances in which torpedo boats can close with larger ships. However, even in WWII in the Solomon Islands chain torpedo boats were startlingly ineffective at actually torpedoing anything, and that's about as ideal an environment as it gets. Radar systems have only gotten better since, and counter-fire more accurate.
But if there was no place for regular torpedo what did the Chinese, vietnamese, NK and east German planned to do with such large inventories of torpedo boats in their navies ?If im.not mistaken and frequently I am I recall Norwegian and Danish navies operated some in the 70s and 80s too
Most East German torpedo boats were either converted to patrol ships by the 1970s or else could swap out the torpedo tubes for mines or commandoes, all roles they could actually do something in.
China kept them around as a swarm tactic for inshore coastal defense. Here they're intended to be covered by other assets. In many ways they're the old "People's War" concept the ground forces adhered to for so long and began to move away from in the 1990s.
North Korea is one of the few countries with an actual use case: tangling with South Korean patrol boats near the sea-based DMZ line.
Most Vietnamese torpedo boats were 60s relics that they held onto for lack of any better options, and when they got their hands on a bunch of OSA II missile boats discarded most of them.
Similarly, Norway's boats were all from the 1950s, before proper antiship missiles, and they switched to missile boats as soon as the Penguin missile was available. Same with Denmark. Kept 'em around because they're small navies that can't afford to be constantly recapitalizing their navies, and torpedo boats are better than nothing.