If I have the Player's Guide and Player's Vault, is there still a reason to buy WoM and SoF? Or is it just duplicating the same material?

These expansion will give you the careers, spells and blessing not found in the core set (or Player's Vault) in components form (cards/sheets)! You may still find these compiled in the tables at the end of your Player's Guide.

If you get the WoM and SoF, you'll also get background and stats for the cults of the dark gods Tzeench and Nurgle, plus an aventure based on each. You'll get some new Tzeench and Nurgle chaos spells on cards too which you won't find in your guides. Also, you'll get a more specialized background book on religion and magic which add a few new rules and content.

Basically, with WoM and SoF you get some talents, some dark magic spells, 20 careers, plus all the spells and blessing for every religion or wind of magic not supported in the core set or player's vault (13 each plus 3 more for the core set one). All of these are in card format. (You've already got these in the compilated table found at the end of the Player's Guide... so you only get new components there, not new content if you don't use components.)

Hope it helps you!

Happy WFRP gaming

PS: Adventurer's Toolkit is the 3rd product were you'll get 10 careers plus some talents and action cards featured in the Player's Guide tables but not included in component form in the player's vault.