Japan is not really a vegetarian oriented place as one might thing. You have a lot you can eat but depend on your location.
In Sapporo as a vegetarian I will recommend Iki Laboratory.
The food is what I like best: home cooking vegan food made with love.
Eating delicious creative food served by an awesome host is like dinning dessert :-) you can taste the kindness in their food.
We really enjoyed our time at Iki laboratory.Pros: fabulous confort food, creative, friendly staff
Cons: not into mock meat