Hi Hoat,
Although I only came to know about you or your site from the SE OpenSocial and Twitter, I think you're a special individual and the oppurtunity to join Google Singapore, although being away from Hanoi is a great chance for bigger things in the future.
Of course there are things to give up and other things to gain in different places, but joining Google which has a lot of resources in terms of technologies and brand, can easily overcome the worries you have in mind I guess?
And of course this is my personal opinion as I have never worked in Google before nor had I visited their offices yet, hence it's just my own views.
However, I do wish you the very best in the choice you make. :-)