TheFork users who ate in Leuven have said really good things about Lalibela, Annapurna and Le Thème Brussels. Maybe you should give them a try!
Some of the more popular Locally produced restaurants that offer vegetarian options on their menu in Leuven according to TheFork users include:
Zita Restaurant, with a 8.5 rating
kalypta, with a 7.5 rating
Lalibela, with a 8.5 rating