See the following snippet:

@file = File.open("doc.txt", "w")
@file.write("Test One ")
@file.write("Test Two")

In RPG Maker VX this would normally write 'Test One Test Two' in doc.txt, but in RPG Maker VX-Ace it just leave the txt file empty... Anyone know a solution for this?

You just forgot to close the I/O stream. Either use @file.close or a block.