Vegetables do not contain collagen; only bones do, making a meat stock reduction gelatinous. Therefore, a reduced veg stock remains quite liquid and never reaches the consistency of a glaze. Not to mention, the flavour is not very good..
You can mock a demiglaze by adding roasted and pureed red beet(and any other seasonings/ingredients, According to your application) to your veg stock and thicken slightly with an arrowroot slurry. I have been using this method for years in vegetarian cooking.:chef:
A vegetarian gelling compound that resembles gelatin the most is Agar-Agar. You can find this in Asian food markets. Agar-Agar is a seaweed extract but is completely tasteless. To dissolve, the liquid must simmer.