We have a niece (15 years old) who is living in Vietnam with the parents, can we adopt her?

It may be possible to adopt your niece in Vietnam, but this would be an international adoption. Although Vietnam has not signed the Hague Convention on Child Trafficking the United States has. As a result, you would need to work with an agency in the United States that is Hague Convention Certified. You can find a list of these agencies at:
http://adoption.state.gov/hague_convention/agency_accreditation/agency_s...

In addition, I would suggest that you visit this web site that is maintained by he US Dept. of Health and Human Services. It provides detailed information about Intercountry adoption from Hague Convention and Non-Hague Convention Countries: http://www.childwelfare.gov/pubs/factsheets/hague.cfm