I'm going to start off by saying this-I'm quite aware that Virgil's lists the spices they use to flavor the root beer on the packaging, however, they do not list amounts. I decided to give it a try, excluding those spices I find either undesirable(cloves), or those things which I did not have in my ingredients vault(birch of any kind).
I combined all of the below together, and came up with a very close approximation of a tasty bottle of Virgil's.
1 tbsp. Licorice Root
4 crushed Star Anise Pods
1 Cinnamon stick(approx. 4 inches)
1 Vanilla Bean(approx. 6 inches)
1tbsp. dried Wintergreen Leaves
1/4 tsp. ground Nutmeg
5 pieces of whole Allspice
2 cups of Cane Sugar
2 tbsp. Blackstrap Molasses
1/8 tsp. yeast
4.5 qts. of Water
Start off filling your stockpot with 4.5 quarts of water, place all ingredients(minus sugar + yeast) into pot, turn to med-hi heat and bring to a boil. Reduce heat to medium-low and allow to simmer with the lid on for approximately 20-30 minutes. Stir in your 2 cups of sugar and molasses, place in ice-water bath to allow to cool.
When the mixture gets down to about 95 degrees Fahrenheit, scoop a little bit into a coffee cup and add yeast to the coffee cup. After allowing yeast to culture for about 5 minutes, dump contents of coffee cup back into pot and stir in well. Bottle and wait 3-4 days(dependent on temperature), then move bottles to the fridge. You'll have a nice, homemade version of Virgil's Root Beer waiting for you. I'd say your best flavor comes about 3 days after placing it in the fridge. The amount of molasses can be modified to taste.
Try it for yourself and let me know in the comments how much/little you think it tastes like Virgil's.