Fins won't play much roll with stabilizing the board, however using a fin that's larger with more surface area will lend a tiny bit. Stability is mainly gained from the width of the board. Since the Vader is narrow, you will have a longer learning curve regardless of what size fin you use.
The Tomo boards can be used with smaller fins due to their more vertical rail profile. The other thing I STRONGLY recommend for the Vader (either thruster or quad set-up) is to use "inside foiled" front fins. Futures called it "V2" and FCS and most others call it "IFT". Tomo boards come alive with these type of fins in my testing and experience.
Any fin on this page would be ideal...
https://futuresfins.com/en-us/shop-surf ... generating
A good "all-around" fin shape is the F4, so it's a good default fin if don't have a preference or specific need.
Since you don't have an established configuration preference, definitely get 5 fins so you can switch it up to see what works best for you or your conditions. Some conditions a thruster can be better, but it's all personal. Many seem to like the Tomo boards better in quad mode fwiw.
Note: You can buy rear quad fins separately, so you can either build your 5-fin set (combining a thruster fin set with some quad rear fins) or you can just buy a pre-package 5-fin set.
Check out Finatic too...