After you paste the text, (or anything previously copied to the clipboard before entering Vanishing Point) into Vanishing Point, there is no reason why you can't use Free Transform to scale it back to your required size, and aspect ratio. If the text or object is too big, and the FT handles are outside the VP Plane, just drag it down so you can see the upper handles, scale it down, and drag it back into position.
Note: that when using FT Perspective on any object (I am talking about outside of VP) it tends to exaggerate the 'length' because that's how we would probably see the object in real life. Perspective I just about the only time I actually right click and chose an option from within FT, because it is so easy and quick to Ctrl (Cmd) drag a corner or center handle.
Unfortunately you only get the basic FT options of size and aspect ratio from within VP, which is a pity because you can't distort your object to apparently wrap onto a curved surface. I added some FT warp to this text before applying it from within VP. I suspect I am missing a trick or two with VP as I don't use it very often, so I'd love to hear from someone who really has it under control?
Trevor, that makes all kinds of sense, except the FT controls a whole lot of just empty space. So, even when I zoom out of the VP preview and expand the plane 600% or so past the edge of the image, I still can't make the text fill up the right area — the text gets crammed way in the back.
This shows the bounding box of the copied text within the vanishing point area: