Sorry about the tardy reply - I've been out of the country and kind of killed my phone so haven't been able to communicate.
Are you using graphics plugins perchance? I know that the Unity engine doesn't play well with many of the more popular Vita plugins and there's no way to detect what could be happening here.
Failing that please check over your install and make sure it's done by the letter as I really tried to make things as reliant as possible on the original Portal game so that this wouldn't be considered to be piracy.
Which brings us to the other potential issue that you could be encountering: The data files were made as a diff against the latest version of Portal available for Windows, so not using that version could be befouling you also.
I hope this helps you track down what the problem is, and good luck!