Vegan actually means eating/consuming no animal products/derivatives, so this could include unhealthy foods. A raw plant-based diet is I think what you meant to say, and I am in agreement with you on that.
Have you tried the Nutritarian way of eating as an alternative?
Juices will allow your body to enter ketosis partially, but not fully as in water-only, and the reason the overweight lose weight on a reducing diet: more calories out than coming in means you burn fat (ketosis).
The Ketogenic diet seems to be in vogue these days in eliminating tumors.
How about having a second or third Medical opinion on the surgery, as what has been recommended seems rather extreme?
Personally, I would opt for veggie juices (esp' organic carrot) and limit the fruit juices.
Sorry, but is the tumor causing any discomfort/symptoms or any disruption to your daily life? Apologies in asking you to repeat what you may have already posted on previously.