Take my word for it.........You would have to own an extremely high end laptop to match most reasonable desktop units for speed.

The issues are as follows;

1. The screen on a laptop will be too small and slow.
2. Sony Vegas has a fiddly and complicated user interface and a large (19" plus) screen will enable you to use it better.
3. For £1500, you will get an extremely capable computer that will process your video's more effectively.
4. Sound systems are another factor. Film production isn't just about the video aspect. The best productions combine all aspects to excite all senses.

If you are doing this for work, why don't you suggest to the powers that be that you work from home whilst producing your work. In my experience, I do the best work in one place. Changing locations can cause continuity problems.

It's like reading a book in your favourite chair. reading it somewhere else isn't quite the same!

Go for the best desktop you can.