I have seen the movie "killing fields" many years back, but I did not know the history then. Vietnam with its invasion of Cambodia removed Pol Pot and Khmer Rouge (Red Khmer), who killed 2-3 million people in their "communist" purges. I thank and congratulate Vietnam for doing this great service for humanity.
Having said that, the world has changed quite a bit since the 70's and 80's. China has changed a lot, so has Vietnam, but the power difference is rising astronomically. I am afraid China's military strength is growing much more rapidly than Vietnam's. This is because of many different factors, but being large and having economies of scale is helping China, while Vietnam is getting behind. Russia is no longer what it used to be, so although the US and to some respect India can also work as a balancer for Vietnam, the situation is not good in the long term, as US is not gaining ground against China and India is not truly a balancer for China, without the US.
This situation is also playing out in East Asia for Japan and South Korea, both of which are under security umbrella of the US and the situation will get more precarious every year.
So I think ASEAN+ arrangement will increasingly look attractive for all three states, actually 4, if we include the Philippines. The formation of ASEAN+ does not mean that individual states will have to leave security arrangement with the US. In fact, I think the US might also be interested to have a strong power as an ally which ASEAN+ have the potential to be. So I am hoping that the US will adopt the ASEAN+ model as an integral part of the Asian Pivot foreign policy direction which I have mentioned in this post:
http://www.defence.pk/forums/world-affairs/180755-geopolitics-asean-region-2.html#post3001475
ASEAN+ member states has slightly more than a billion people, the land mass is slightly less than China and current GDP also far exceeds China. If the integration is done in a systematic and proper way without mistakes, the growth of this area can be high, which may keep it ahead of China in many respects.
The Chinese should not look at ASEAN+ as a threat. As a close neighbor, both China and ASEAN+ will have the closest of economic relations and will be dependent on each other. The security part of course will be separate and distinct for both. The interesting part for China will be that as ASEAN+ matures, it will become less and less dependent on the US, which will mean that the US bases in this region may not be necessary any more at some point. That will be the point when ASEAN+ will hopefully start charting a more independent course and may even become friendlier toward China, as China afterall is a neighbor, while the US is a distant power with not so much in common.
As for the overseas Chinese, lets look at some numbers:
Han Chinese - Wikipedia, the free encyclopedia
Overseas Chinese - Wikipedia, the free encyclopedia
This is a very influential group of people in ASEAN+ group states. Instead of seeing their influence as a problem, I believe it is possible to turn this reality into an opportunity, by utilizing them as a lobbying group for ASEAN+ with the Chinese in China proper (China+Taiwan). After all, if ASEAN+ prospers, they prosper as well. ASEAN+ security and stability, is their security and stability as well.
While as an ASEAN+ state, Singapore has every right to be involved in ASEAN+ affairs, I do not think the same is true for China or any other state that is outside the group of member nations.