Yes, definitely install VI! I waited because I enjoyed MTW just the way it was, but after I installed VI, I enjoyed MTW even more! I rarely play the Viking campaign anyway, but I'm grateful for what VI has given to the MTW play.
Pre-battle: after you invade or are invaded, instead of going right to the battle screen, you are given a "pre-battle" screen, which lists all of the forces you and your opponent have ready to fight. You can tailor your force (swap out/in units) to match/counter what the AI has, or, you can elect to withdraw entirely.
There are some other changes that VI adds to MTW like more units and playable factions, but the pre-battle screen makes you WANT to head into combat.
Don't delay, me-lad!