With that setup, I can't see why you should even be having a problem with the lite version. (There is currently an issue with PHP 4).
To answer your question, the paid version is coded significantly differently than the lite version. It is quite insular, such that it automatically disables and doesn't load the individual parts of itself that it doesn't need.
However, there are portions of the product that remain the same. Without further investigation of how the code reacts in a similar environment to yours, it is very difficult to say whether the problems you are having are unique to the Lite version, your setup, or due to a conflict. If it's possible for you to set up a private test bed, I would like to investigate this further (would need edit access to the test /vault directory). I think this is the best route to take, rather than having you spend the money first and discovering later the same problem is occurring.