This question is not so black and white as is implied.
Generally, one can find 4 types:
Viet Kieu that criticize Viet Nam
Viet Kieu that don’t criticize Viet Nam
Domestic Vietnamese that criticize Viet Nam
Domestic Vietnamese who don’t criticize Viet Nam (although this is rare indeed)
Criticism of Viet Nam can be multi-dimensional; Politics, Economics, Social Conditions, Environment…really across all Environments (Legal, Technological, Environmental, Social, Economic and Political; or Even Demographic, Financial)
But generally, the issue behind this question is Government and Political or Legal Issues. But for some can also highly involve Economic or Financial and Social. These intertwine. For example, government policy on education, and the control of it, that holds back entrepreneurship that could help advance the skills of the Vietnamese. Sometimes real issues, far beyond the political for some, with real implications for Vietnams development. Or the degree to which Civil Society organizations can organically form to develop skill-sharing networks across the country to promote best practices in social, agrarian or industrial fields with ability to openly discuss associated issues and promulgate perspectives that can help increase efficiency and proficiency of actors and agents in the society..
But, whether criticism exists, and that there are those that are bothered by it, or not, involves a range of Psychological factors. These would range from beliefs held, true or false, and the range of cognitive and sense-making factors involved in creating and holding a belief.
Aspects of the issue could be jealousy on the one hand, or could be mere wearisome on the other. Some may be jealous of the material prosperity of their foreign-domiciled aunts, uncles and cousins. On the other hand, some may be tired of all the political issues and merely want to see the country develop and prosperity to rise. Some may lament what was taken from their families (those inside and outside the country) and be critical. Some may have moved past and understand that perfect situations are not achievable but one need be more pragmatic abut the situation from today to tomorrow.
The Vietnamese are a remarkably open people. They, unlike many others in Asia, while being vehemently nationalistic, are more culturally open to new ideas and ways. They are far less culturally boxed-in then North East Asians. And they hold, and openly discuss, a wide variety of perspectives. It is how one acts on those ideas that can cause problems.
I have seen disagreements between Viet Kieu over this very issue. Really depends if one is looking at yesterday or tomorrow. But all of them, want a better tomorrow for the homeland of their birth. I suspect that if change ever comes to Viet Nam, that they are a country that can handle the transition well as they maintain individuality and a spirit for a sense of a community in a way I find unique regionally/globally.