Change US Army to United States Army will clear the suggestion by putting the profile into the [[Category:United States Army, Vietnam War]] which is not a Top Level Category.
If the branch= parameter isn't one that is recognised by the sticker it does default to the highest level. https://www.wikitree.com/wiki/Template:Vietnam_War shows the accepted parameters for this sticker.
Every sticker would have a Template page that can be found in the category https://www.wikitree.com/wiki/Category:Stickers which can be found under the Help option in the drop down menus then going to Templates and then to Stickers. This will show how to use the Templates in an Example Section which will show if a Sticker generates a category or not. Because each Sticker can generate the category different ways there isn't one way to assume it will work.
For the Vietnam sticker for example it splits the end category into three parts. Using the example on the Sticker page of [[Category : 1st Marine Division, United States Marine Corps, Vietnam War]] the |unit= parameter is entered as 1st Marine Division, The |Branch= parameter is entered as United States Marine Corps and the Vietnam War part of the category is generated from the Sticker Name.
For the Roll of Honor sticker on the other hand you would need to use a |Category= parameter that has the full category name such as Killed in Action, United States of America, Vietnam War to generate the right category.