Thank you for replying! Let me explain what I did...
I just added client login to SmallHost, thanks for your help! I'll share a public version of the source code for that later. Basically the user logins with their info and SmallHost connects to the FTP server to check if the info is correct. If it is, it creates an encoded cookie with that information. I then use the encoded cookie throughout the client panel (for example, so the users can access the control panel or file manager with a click of a button without needing to keep logging in). The client panel is now online, and I even sat up a way that users can sign in on mobile by scanning a QR code. I'm still working on making SmallHost better!
Regarding making a direct login to the file manager, I did that prior to working on a better client panel. I realized that the link when you click the File Manager button on the vPanel uses an encoded version of your password. Using this tool, I decoded it and made this code (you can use it, if it's helpful)...
<?php
$pw = '{"t":"ftp","c":{"v":0,"p":"'.$_POST['passwd'].'"}}';
$encodePw = base64_encode($pw);
header("Location: https://filemanager.ai/new/#/c/185.27.134.11/".$_POST['uname']."/".$encodePw);
die();
?>
The client panel is now up and running, however I'm still working on making it better and adding new features. Thank you all for your help!