I have been looking around for quite some time searching for PHP platform that serves the following requirements:

1.Install different PHP scripts such as VCards, VBulletin, Links, News…etc. from different venders. However; all of these scripts will use MySql DataBase.
2.One single user account to access all different scripts.
3.Can I assign single user with different access level for each script, or develop a users grouping for that purpose.
4.Preferable to have one database for all different scripts, but not necessarily.

Any suggestion will be highly appreciated.

called, "do it your self"

Quite funny... Such specific requisites would 110% probably mean that you're either going to have to write it yourself, or hire myself or others to do the job.

You can use one databsae and have each script with a PREFIXED table names.

FOr instance the script using DB, MyDB and its fields named

script1_table1
script1_table2
etc etc..

that way you can use 1 database for all scripts.

ALot of php script have Setup and Config files that allow you do add the prefix for this very reason.

Otherwise just edit the PHP script yourself since its all open code.

Your best bet will be to...

1.) Find a CMS that supports as many of your favorite third party scripts as possible.
2.) Code it yourself, and with requirements like yours, you better be really good and have a whole mess of time.
3.) Find someone else who matches the above description and be willing to part with bunches of cash.