I want to record in audacity the audio of a video file (song) on vlc and synthesiser (through audio in in the computer) at the same time. is it possible? OS is windows 8
Let me read this back to you. You want to capture the sound portion of a video which a lot of people have trouble doing and capture a live analog performance which in Windows is very difficult and do them both at the same time, which is impossible.
I would do it by capturing the VLC movie as a separate event.
Or sweet-talk VLC into giving you the video sound file. Consult your instructions.
Play your music device into the computer. If you have a Windows laptop chances are zero you have a stereo analog connection, so you will need something like a Behringer UCA202 stereo USB interface.
While you have all that connected, play the video sound file in Audacity and set up for overdubbing, where you can play along with the sound file with your keyboard or music device.
As far as doing it all live, I doubt it unless you can get something like Voicemeeter to work.
Typically, Audacity doesn’t get along well with other sound programs trying to run at the same time.