The Vietnam War occupies a pretty unique and deeply contested place in American popular culture and in the US popular memory, for a whole laundry list of reasons. This popular memory, and the general attitude towards the war and Vietnam Veterans, has also shifted significantly over time, particularly (at least anecdotally) in the wake of the Gulf War, but also with the arrival of refugees in 1976, debates over the anti-war/counter-culture movement, Agent Orange litigation, the POW/MIA issue... the war even became a major Presidential campaign issue over 30 years after the US withdrew.
How have the countries in the former USSR dealt with the legacy of Afghanistan, if they have done so at all? I understand this is an incredibly broad question, since it covers 15 different countries, but the interplay between Afghanistan and the collapse of the USSR makes this issue particularly interesting. I am vaguely aware of major issues with substance abuse and a lack of state support for Afghan veterans in Russia at the very least, but this does not answer how the war is remembered (if at all). Did these perceptions change in the wake of the Chechen Wars or other events?
I know these processes of historical contestation are always ongoing, and while I think school curricula and historiography are sort of an exception to the 20-year rule, I do want to throw out an extra reminder to be mindful of that (OTOH I am curious about what impact the Donbass War would've had on all this, but that is still ongoing and I have no problems re-asking this in 15 years or so for that purpose). As a final note: I do not wish to overlook or erase Afghan perspectives or memory of the war, but that seems like a subject that deserves its own question, especially since the Saur Revolution and Soviet invasion were the first round of over four decades of war.
This is a fascinating question, and I can provide a sliver of an answer by focusing on the Russian memory of the war.
An important backdrop is probably the 1968 invasion of the Czech Republic, and the complicated legacy ten years on (especially in light of the American intervention in Vietnam). The Brezhnev era is often characterized as 'stagnant,' derided by Gorbachev (and inclusive of Brezhnev's successors).
In the night on December 24, 1979, Soviet landings began in Kabul and Bagram. Earlier that year the Soviet Union had expressed reticence at even entering, and in 1979 proposed only to support the Afghan army. But this intervention quickly devolved, and by the conclusion of the war between 14,000 and 15,000 Soviets were killed (nearly 60,000 Americans were killed in the Vietnam War), Upwards of two million Afghan citizens were killed in the conflict. When the Soviet Union ultimately withdrew from the conflict, internally it was seen as an admission of military weakness, and threw into question doctrines of intervention, the tenability of the Soviet state, and the Soviet place on the world stage.
Cultural reflections on the war are, just like for the American case of the Vietnam War, varied. A recent film that offers a sympathetic and complicated portrayal of the war is Bondarchuk's 9th Company, which is set in January 1988. The 1994 film Peshavar Waltz is an interpretation of the Badaber uprising of 1985 (where all Soviet and Afghan prisoners were ultimately killed), but I think doesn't quite capture the broader experience the way 9th Company might. Certainly the Russian response to young soldiers in Chechnya was quite different from the early response to the same in Afghanistan, and I think that the reflection in the late 1980s contributed to this reassessment, but I don't think the case is entirely analogous to the American opposition to the Vietnam War for its entire duration.
The transition from the exuberance (or understated promise, depending on actual events on the ground) from the early years of the conflict to the more tempered media analysis under Gorbachev is one of the major reflections of glasnost' - in 1980, for example, soldiers and veterans were disallowed from attending the Summer Olympics out of concern they would disseminate information about actual conditions. But not all the new information of the late 1980s characterizes the soldiers as victim; experienced soldiers are often accused of undue harassment, while incidences of rape, murder, and especially drug abuse are common.
Kino's 1988 Gruppa Krovi is the de facto anthem of the conflict, by then well-recognized by Russians for the reality both of state power and of the individual lives of soldiers - in the title song, yearning to stay so they need not 'place their boot on someone.' The title refers to the blood type marked on soldier's sleeves; a stark, dehumanizing reminder of the soldier's role.
Rodric Braithwaite's Afgantsy is remarkably readable and gripping, and focuses on exactly the kinds of questions you propose (substance abuse, especially heroin, and faltering state support). With the twenty year rule in mind, by the late 1990s the Russian reflection was melancholic and regretful (at the very least, the aims of the Soviet intervention certainly had not succeeded, and probably could not have), but more recently this assessment has shifted despite the material outcome for the Soviet Union and Afghanistan. Only recently, for example, were veterans honored for their service, but this has been done in a celebratory fashion.