Encouraged by your hint, I add the following to my search module (customized for my theme), and it looked great, but it doesn't return any results for me even when I search authors by my own name ...
Code:
<tr><td class="tcat"><div align=center><font size=2 color=red>$vbphrase[vbart_searchart]</font></div></td></tr>
<tr><td class="alt1" align="left">
<form action="articles.php" method="post" enctype="multipart/form-data">
<input type="hidden" name="action" value="dosearcharticle" />
<input type="text" name="keywords"><br>
<select name="searchby">
<option value="searchall" selected>$vbphrase[vbart_searchall]
<option value="title">$vbphrase[vbart_searchtitle]
<option value="content">$vbphrase[vbart_searchcontent]
<option value="author">$vbphrase[vbart_searchauthor]
</select><br><br>
<input type="submit" class="button" value="GO" accesskey="s" />