Libre Office is independent of desktop environments, so you can run it in Lubuntu and Xubuntu. You need not run Ubuntu with Unity, if that desktop environment needs more horsepower than what is in your computer. Instead you can backup your system or at least your personal files and
- install Lubuntu or Xubuntu and install Libre Office into that operating system
- install a light desktop environment into Ubuntu and select it at the login screen. Click on the round symbol near the window where to enter the password to select 'session' alias desktop environment!
sudo apt-get install lubuntu-desktop
or if you only want to desktop environment (without the full Lubuntu or Xubuntu specific software and tweaks)
sudo apt-get install xubuntu-desktop
sudo apt-get install lxde
sudo apt-get install xfce4
Maybe it helps to remove and [re]install VLC. But if mplayer2 works well, isn't that enough?