I think you're getting a bit confused here. There are two options for VCentre - On windows or on an appliance, the VCentre install on Windows is largely end of life so you're better off installing the appliance.

As for the thick client (Sometimes called the C# client or the installable client), that is also end of life.

Now, there are also two web clients - the flash version that has all the functionality of the thick client and is horrible to use and the HTML 5 client which has 80% or so of all the functionality of the flash version and is simply gorgeous and lovely.

To complicate things further - the HTML 5 client is only available on VCentre 6.5, it's possible to use on 6.0 via fling (VMware beta type software). It is not possible to use on 5.5 which only has the flash client.

In short, move to 6.5, install the VCSA appliance and use the HTML 5 client for everything that you can. Use the flash client if you must.