My party has a Barbarian, Samurai, Paladin, Rogue, Monk, Warlock. All around level 7-9. When if ever is a good time to change class? I did not take the Vine magic at the beginning. Now I have to olad up on poison potions. Can I still get it or do I have to Become a Ranger?

Basic rule of thumb is that you can change fighter classes to a new class early - level 7 is fine. Magic users it seems best to wait until they have reached level 14 and have picked up the best of the best spells available to their class before changing them to another.
Rangers are a pretty good class and if you want to up your Vine magic, that might be the most efficient way to do so.

Hope that helps, Magnus!

Wyv