There is a table of amino acid composition in beers on page 105 of Beer: Health and Nutrition (C. W. Bamforth, 2004) (page 119 of the overall PDF):
Other than that, I cannot find any nutritional breakdowns that include vitamins for stouts (and porters, in general). However, as far as micronutrients are concerned, there may not be as great a difference between stouts and other ales as one might think, since, despite the perceptibly large differences in color and taste, the ingredients and processes in brewing are largely the same. Here are some findings on beer in general:
Querying alcohol on USDA's National Nutrient Database yields breakdowns for
- Alcoholic beverage, beer, light
- Alcoholic beverage, beer, regular, all
- Alcoholic beverage, beer, regular, BUDWEISER
- Alcoholic beverage, beer, light, BUDWEISER SELECT
- Alcoholic beverage, beer, light, BUD LIGHT
- Alcoholic beverage, beer, light, MICHELOB ULTRA
Here's a screenshot, because the host doesn't always resolve for me for some reason:
I'm willing to bet that The Irish Food Composition Database has a similar report on Guinness, but the document must be purchased for 15.00€ from EuroFIR. According to a table of contents viewable in the demo link, the document should contain compositions of 57 manufactured beverages.
Perhaps you can infer some properties of Guinness by knowing its ingredients and process and also knowing how these ingredients and processes affect micronutrient levels.
According to this paper1, as far as major B vitamins are concerned,
beer contains very little thiamine (B1), of which malting the barley causes a slight loss, but otherwise stays intact through the rest of the brewing operations,
riboflavin (B2) increases during the malting and brewing processes, resulting in nutritionally significant amounts (e.g. seven pints supplying total daily requirements),
nicotinic acid (niacin, B3) fluctuates throughout the malting and brewing process, but also results in nutritionally significant amounts (e.g. three-and-a-half to four pints supplying total daily requirements).
This study2 measured phylloquinone (K1) composition of beers, specifically a "bitter," a lager, and Guinness, and concluded only trace amounts in each.
1 Stringer, W. J. Vitamins in Beer. Journal of the Institute of Brewing. 2013.
2 Bolton-Smith, C. et al. Compilation of a provisional UK database for the phylloquinone (vitamin K1) content of foods. British Journal of Nutrition. 2000.