If you are webmaster, you can demote the unwanted content using google web master tools.

Configuration -> Sitelinks-> Demote this sitelink URL:

And obviously, keep updating robots.txt moving forward.

Please see https://support.google.com/webmasters/bin/answer.py?hl=en&answer=47334

You can demote up to 100 URLs, and demotions are effective for 90 days from your most recent visit to the Sitelinks page in Webmaster Tools.

If I were you, then on http://sandbox.vietnamvisacorp.com/  I would put a 301 redirect based on ip address on all pages.

This will redirect all users to http://www.vietnamvisacorp.com/ based on ip address, i.e. if IP address = your ip then don't redirect otherwise redirect.

Hmmm.. I think its the IP address of your computer i.e. goto http://whatismyipaddress.com/  and get your IP address and use it below.

RewriteCond %{REMOTE_HOST} !^123.45.67.8
RewriteRule (.*) http://www.vietnamvisacorp.com/$1 [R=301,L]

I am sorry, not much of an Apache expert.