What is the purpose of vermouth in potato leek soup? I have taken elements of a number of different recipes and have developed my own recipe for potato leek soup.  The ingredients are:


Potatos
Leeks
Vegetable and chicken stock
Butter
Garlic
Heavy cream
Salt, various herbs and spices
Vermouth


Quite a few of the recipes I built mine from included the dry vermouth so I built it into mine as well.  In an 8 qt stock pot I use 2/3 cup (specifically Martini &amp; Rossi Extra Dry).  It's not enough for me to notice a flavor difference between batches that I make with it and without.  That leaves me to wonder what it's there for.  I guess it could provide a touch of sweetness.  Would it be for the acidic factor as well?

If you need more detailed information on the ratios of ingredients I'd be happy to share, just left it out for brevity.
Vermouth is a fortified white wine that has been infused with herbs, spices and fruits. You should expect it to add those flavors to your dish. 

Have you tasted the vermouth on its own? Are you sure you can't identify it? If you still think you can't, I'd suggest tasting your soup as you add each ingredient and concentrate on what flavors each ingredient adds.

For what it's worth, Cooks Illustrated has reviewed Martini &amp; Rossi Extra Dry and classified it as "not recommended" due to tasting medicinal and harsh.

Gallo Extra Dry California and Noilly Prat Original French were both recommended, in that order.

They used a pan sauce as well as straight tasting for their reviews. Perhaps you could try that yourself and see what flavors vermouth gives to a pan sauce, as compared to a white wine. You might find out that you're overpowering or masking a flavor in your vermouth with some of your various herbs &amp; spices.