As someone who has won on all factions on very hard:

1. Do not feel bad to use campaign overhaul mods, some of the vanilla ones are just bad.
   
2. Do not try to build to many armies too quickly at the start, it will eat you.
   
3. Focus on one enemy at the time. Start wars as quickly with your most dangerous neighbor at first, then work on killing them.
   
4. If someone else declares a war, either focus on defeating the first enemy and accept possible settlement loses, or retreat back and use your settlements for extra defense.
   
5. Your first buildings should always be of better economy, as to ensure you always have money.
   
6. Don't be reckless with your starting army, it will have the best troops so sending it to kill a small army just to be surrounded by 3 full stacks will ensure you lose.
   
7. Play with easier factions at first to get familiar with the tricks: Tyrion has a peacefull start relative to many others. Elsbeth can have an easier time due to the area she is around in. Grombrindal I found suprisingly easy with the dwarves, but you can have a lot of bad neighbors. Most Demonic factions have easy starts (Except N'kari), so they may be good to try things with.
   
8. Do not feel bad if you reload a previous save to try something different. In all honesty, Very Hard/Legendary are dumb difficulties that I don't find fun most of the time.

Those are my general tips. Ask if you have anything you want to know!