[Tex/LaTex] How to change the name of Index section

Question

I use the makeidx in the extreport and the command \printindex to print the index out. The Index name (index in English) was translated to Vietnamese automatically into "Chỉ mục". I want to change "Chỉ mục" into "Tra cứu theo vần", which is much easier to understand in Vietnamse, my native language. So how can I do?

This is my MWE :

\documentclass[11pt,twoside]{extreport}
\usepackage[paperwidth=16cm,paperheight=24cm]{geometry}
\usepackage{amsmath,amsxtra,amssymb,latexsym,  amscd,amsfonts,enumerate,ifthen,stmaryrd,amsthm,amstext}
\usepackage[mathscr]{eucal}
\usepackage{graphics, graphpap,extsizes}
\usepackage[pdftex]{graphicx}
\usepackage{xspace}
\usepackage{mathrsfs}
\usepackage{makeidx}
\usepackage{indentfirst}
\usepackage[utf8]{inputenc}
\usepackage[utf8]{vietnam}
\usepackage{perpage} 

\makeindex 

\begin{document}
\section*{Chuyển sang tứ giác} \index{hình vuông} 
\index{đối xứng trục}       

\index{đối xứng quay}

 Nhóm đối xứng \index{Nhóm đối xứng, i.e the symmetric group} của một tứ giác. 
\index{how can I do?} Ok let's see. 
\newpage 
\printindex
\end{document}

Here is the picture :

Answer 1

You can either put

\renewcommand\indexname{Tra cứu theo vần}

after \begin{document} or

\AtBeginDocument{\renewcommand\indexname{Tra cứu theo vần}}

in the preamble.

Note that placing the former in the preamble doesn't work since vietnam.sty has a line

\AtBeginDocument{\captionsvietnam\datevietnam}

which loads vietnamise names...