If you see a DisplayPort connector on the laptop, then your laptop has the hardware to support DisplayPort output, and yes you'd be able to use VGA and DisplayPort at the same time. However, in that case I don't think you'd also be able to use your built-in display as a third panel. I'm fairly sure the E6400 only supports 2 displays simultaneously. I'm also not sure what revision of DisplayPort that system supports, which means you should definitely NOT count on MST/daisy-chaining support, and higher resolutions like 4K and even 2560x1600 might be off the table. If the latter is a concern, you may be able to find information about the maximum supported resolution of that system elsewhere. The E6400 was a widely used laptop, so somebody will have tested at least 2560x1600.