Way too broad of a question. I suggest you do a lot of reading about Vietnam and then ask a more specific question.

augusta1 is correct. You may want to look at the 1979 border war with China, then the Subsidized Period, and of course the advent of doi moi policy. These topics should help you trace some of the major events post-75. You sound mostly curious about how the Vietnamese have a high opinion of the US. If this poll asked about US foreign policy, they'd come up with a different result. Those Viets who supported the North naturally enjoy the fact that they won, so their approach is - the past is the past, what's the big deal? The Viets who supported the South will tell you they still love the US. It's not by any means a completely positive view of America across Vietnam, but, especially in this age of China rising, the US seems like a much lesser evil. Good luck with your research and question.

85 % of Vietnam's population is under the age of 40. 60% is under the 30. That means the vast majority of people have no memory of the war. Their first memories are of the post-war period. What was that like? The SRV (communist government of VN) sent hundreds of thousands of people from the south to reeducation camps. Many were released quickly, some spent over a decade confined. Others died in the camps. Meanwhile, the state directed collectivization program destroyed the more prosperous southern economy. It was a time of terrible privation. Again, thousands risked their lives to flee. Eventually the SRV offered former South Vietnam business officials their land and positions back at banks, etc to rebuild the economy. The Đổi Mới reforms followed after 1986 and brought a liberalization of the economy. The normalization of US-SRV relations in 1995, and resumption of trade, was very popular. Inflation is still sky high, and you are prohibited from buying foreign currency. One dollar? That's 21,000 đồng.

In the south, many people look at the trajectory of South Korea, whose economy began to grow exponentially in the late 1970s, after the war was already over. They look at this with regret, thinking that a southern Vietnam state, had it survived, would be much better today. There's quite a contradiction between the rhetoric of North Vietnam during the war and how it conducts itself today. No more about collectivization and redistribution. Now government officials hoard as much wealth as possible. Guess who owns the rights for the first McDonald's franchises that opened in Vietnam last year? The Prime Minister's daughter and husband! Add to that most Vietnamese who fled the country are thriving in the US, Australia, France, and elsewhere. Often they are sending money home to support relatives.

And of course, as someone else mentioned, the rise of China has produced new problems as it tries to expand its influence. I've often had people ask me if I was American, then respond, 'good, I hate China! I hope Vietnam allies with the US!'.