By roasting a chicken you already have all the ingredients for a rich and delicious gravy.
The need to create an amazing reduction, which is often my first choice when it comes to a massive impact of flavor, and a heady delight for the pallate.
Once the bird is finished, remove it from the pan and deglaze the pan with a little white wine, in your case vegetable stock will work very nicely.
Over low heat, use a wooden spoon or other implement that will not damage your cookware and gently scrape all the brown bits and caramelised goodies from the pan and work them into the liquid.
The broth can be poured into a saute pan at this point to improve the efficiency and speed of the final process.
Raise the heat to a rapid boil and while frequently stirring, reduce the mixture by two thirds.
Taste along the way for intensiy as the reduction will become quite potent, and do your seasoning at the end.
If you find the flavor too intense, just add (mount with) tiny ammounts of vegetable stock until the taste is to your liking.
If you really must have a Bechamel like texture to your sauce, you might want to look into using a product like Agar Agar however, be careful to use very small amounts lest you end up with pan dripping Jello to serve along side your beautiful bird.
Who knows, it might be a terrific side dish!
Note: I am afraid to lose this post again, so for intel, google... diabetic concerns with agar agar
You will find everything you need to know to put your mind and tummy at ease.