Connecting an Already Opened Project to Perforce

Is there a way to tell P4VS that the currently opened solution corresponds to a given checkout?

My problem stems from the fact that I'm using a 3rd party tool to open Visual Studio and attach it to a process. But once it's open I want to be able to use the P4VS plugin.

Is that doable, or do i just have to work without P4VS :(

Yeah you can do this, you'll just need to use the "P4 Connection" Toolbar:

You can enable this by right clicking on vacant toolbar area or a toolbar handle in Visual Studio, and checking it.

Once you have the toolbar just log in and supply your current checkout.

If you're going to be doing this a lot it may also be helpful to you to select: "Connect to the server using my most recent" in the "Perforce - Connections" Options so you don't have to keep logging in: