You can't download Java to PS Vita because it's already using C# as the main programming language. This will never be implemented by Sony due to the Oracle's licence, so Sony choose XNA framework along with C# integration and support.
but what to do if i want to play something like a video it dosent let me it says that i need a java/adobe flash players
The vita browser also doesnt support java, it never will. Unfortunatly that means you willl have to find an alternative method.
Hope that helps, JJS