I was reading recently about the Đổi Mới reforms introduced during Nguyễn Văn Linh's tenure as general secretary in the late 1980s, and wondering why Vietnam, based on a Soviet model of socialism, was able to transition to capitalism without collapsing, while the contemporary Soviet perestroika and glasnost led to the nation's collapse. Cuba entered the "special period" in the early 1990s, suffering near famine until the government accepted generous economic aid from the USA and made some reforms, but Vietnam avoided this, despite having previously also been dependent on Soviet aid and economically struggling in the mid 1980s.
You're conflating economic reform with changes to political structures. The Soviet Union "collapsed" because it gained a leader that was no longer willing to enforce the party's monopoly on power, not because it attempted to reform its economics. Yes, the economic aspects of perestroika (a communist-based reform intended to revitalize the traditional command economy, rather than an attempt to introduce wholesale capitalism) were not particularly successful and hurt the already lagging Soviet economy, but many states have gone through rough economic periods and not collapsed. There was nothing necessary or inevitable about the specific reforms Gorbachev passed: they were on his initiative. In a classic contrast between the Brezhnev Doctrine (seen in action just 20 years prior, in Czechoslovakia) and the "Sinatra Doctrine", Gorbachev refused to forcibly intervene to assist the wavering Eastern Bloc. This in turn raised hopes that he would have a similar laissez faire approach to internal Soviet matters, which he did. The 1989 elections saw many separatist candidates win over CPSU candidates, but it is more significant that they were allowed to stand for office in the first place: this was due to Gorbachev's new political policies. This new generation of separatist leaders then declared sovereignty for their various republics, and again Gorbachev chose not to intervene with force.
To stick to the very recent past prior to these decisions, Chernenko, Andropov, or Brezhnev never behaved in the same fashion, with Brezhnev being responsible for the end of the Prague Spring, and Andropov being one of that invasion's strongest supporters, with a notoriously hardline opposition to expressions of dissent in general.
First, the USSR under Gorbachev in no way tried to transition to capitalism. Gorbachev was, by all counts, a committed communist who believe that the USSR needed reform but that reform could make socialism* work.
So that means your question has to be either (A) why did the soviet union suffer a political collapse under Gorbachev or (B) why was Vietnam able to avoid political collapse when so many other communist states did not. I can help with the former question, not as much with the latter.
*The USSR always claimed to be a socialist state that was building communism, but I don't want to get bogged down in the details of this dispute.