Both stopped being colonies around the same time and Vietnam spent 30 years in almost non-stop war like Angola and Mozambique. I used to think that it was because SE Asia had a more complex civilization back in the day but this sub taught me about the civilizations of East and West Africa (in fact Ethiopia was barely even colonized if at all). So what is it then?
You may be interested in /u/khosikulu's contributions to Why are former African colonies generally much less developed than former Asian colonies? and in this thread on why many African countries have struggled in modern history.
I am sorry, but I don't believe that this question is answerable on this sub.
First off, it is a very complex question. What is "development", precisely? Is it simply how wealthy the people of a country are (gdp per capita) and percentage living in poverty? Does it include stuff like literacy rate, secondary school graduation, infant mortality, life expectancy, electrification, ownership of consumer electronics, internet access? Does it include "fair economic growth" and low inequality (and do you measure that by GINI scores?) An economist might be able to write about specific factors that limit economic growth, or why a country has a specific GINI score. A scholar of health care policy and the AIDS crisis might be able to explain that aspect. But it will take someone with extremely broad expertise to be able to talk about how each of these factors is interrelated.
Also. Subsaharan Africa is a region with 46 countries. The developmental history of DRC is not the same as South Africa, which is not the same as Ethiopia, which is different from Zambia. There are some broad, continent spanning trends, but there are also individual policy choices and economies that are outliers on the subcontinent.
That said, say I were to give a broad overview of African economic and development trends from the 1960s to 2000. I don't know about the relevant information for Southeast Asia. I can't say how Southeast Asian policies or interactions with Western or Eastern economic developmental institutions compared.
Finally, just by doing some quick googling of GDP/capita, at first glance it appears that much of the Vietnamese, Indonesian, Cambodian, Thai, etc economic growth has occurred since 2000. So, I think it might be possible that the divergence has happened within the past 20 years (but again, I don't know much about Southeast Asian development). This sub has a 20 year rule. To adequately answer the question, any answer would have to talk quite extensively about events of the past 20 years.
So, sorry, but I don't think this question is answerable in this sub.