Don't think it really helps against DOS and depends on the type of Vit E you've purchased. Even when canola is saponified super fresh and in combo with oils/butters with a long shelf life, there's always a DOS risk...
Tocopherol is used for antioxidating purpose at 0,05-0,1 %, Tocopheryl acetate at 0,5-1%. But you could use more without any consequences.
Supposedly, BHT does a better job, so maybe it's woth looking into that for you :wink:
I have used canola several times and my soap does really well. Here's a recipe:
40% olive
20% coconut
20% palm
10% canola
5% jojoba
5% castor
Good luck!