I would have really struggled as a vegetarian never mind vegan after weight loss surgery.
Couple of challenges - my DP is vegetarian and I can't get enough protein in on his diet in the quantities I can stomach.
I wouldn't want to eat ultra processed food after surgery (evidence suggests this is a leading cause of obesity) which rules out anything that isn't a natural plant on a vegan diet (so no Quorn, other fake meat, vegan 'milk' 'cheese' and 'yogurt' - all highly processed and lower in protein anyway).
The highest protein foods are often unprocessed or minimally processed and low calorie - fish and meat basically.
For me it's about protein Vs calories and the amount I could stomach. For example to get 20g protein in you, you could eat:
90g of cod for 90 calories
70g chicken for 107 calories
Or
A whole can of baked beans 336 calories
Packet of puy lentils 358 calories
You could eat two large portions of Quorn pieces but you wouldn't be able to stomach it and it's Frankenstein food.
Dairy milk is lower calorie and 3x the protein for the same amount of oat milk which is often made with oil (Oatley).
If I was you I would have the surgery, eat meat fish and dairy and consider going back vegetarian or vegan in 3/4 years time. Or at least seek really good nutritional advice. Good luck!