To work with DirectAdmin an FTP server will need to know where to get the user/password files. DirectAdmin doesn't keep everything in one file.
So you'll need to be sure your ftp server of choice will work with DirectAdmin you'll need to verify that it can work with the DirectAdmin paths to the various user/password files used by DirectAdmin.
The last time I used vsftpd (many years ago) it didn't block ftp users from looking into other directories, but relied on system permissions to do so. ProFTPd, my system of choice many years ago, did, and I and many others (including DirectAdmin) switched to it mainly because of it's own permissions granularity.
Pure-ftpd is a drop-in replacement for DirectAdmin; you just change some settings in CustomBuild 2 and install it. Pure-ftpd works well for me and I have no problems with either ProFTPd or Pure-ftpd.
Since you don't tell us what problem your client is having with ProFTPd we can't tell you if any of the other ftp daemons are going to mitigate them. Unless you ask for specific advice about specific issues, you're on your own.
Jeff