I setup an Alias inside of a VirtualHost container for "myothersite.com" which is located:
/home/mypath/to/myothersite.com/httpdocs/ezedit/

Example Alias in vhost.conf:
Alias /ezedit /home/mypath/to/mymainsite.com/httpdocs/ezedit/

For some reason I cannot figure out, instead of executing the index.php file inside of the ezedit directory, my browser prompts to download a file as "ezedit," the directory name, which when downloaded contains the full source of the index.php file.

I have tried changing the php open_basedir, but that did not help. I know this can be done on my server because /webmail, /icons, etc., where already setup on installation. I can't duplicate how they work though, and I can't figure out why.

Any assistance would be greatly appreciated.

try puttin a slash after /ezedit

Alias /ezedit/ /home/mypath/to/mymainsite.com/httpdocs/ezedit/