Reading Executable Files

There are at least two distinct types of .vlt files per Google. What program created the ones you are interested in?

And what are you trying to do opening executable files (what kinds?) in NP++?

One possible way would be to open the file in NP++ by first opening NP++, then using the File Open dialog to browse to the file. Under Plugins, you will find the Hex Editor, which will allow you to examine the file as both Hex numbers and ASCII characters. Be especially careful you don't modify or delete any bytes, as this could cause the executable to become corrupt.