a remaining issue is that the capitalization on your <my:DealerID> tags is incorrect
This I saw, correcting it did not help. Though, honestly, I forgot to recall that capitalization makes difference in XML, so I eventually neglected it
The reason the code isn't working is that the my: namespace in the XML you're constructing is different from the form's my: namespace.
Well, in other words, the article
should have mentioned to replace http://schemas.microsoft.com/office/infopath/2003/myXSD/2008-06-03T00:09:57 in
string vehicleXml = "<my:Vehicle xmlns:my='http://schemas.microsoft.com/office/infopath/2003/myXSD/2008-06-03T00:09:57'>"
+ "<my:Year>" + year + "</my:Year>"
+ "<my:Make>" + make + "</my:Make>"
+ "<my:Model>" + model + "</my:Model>"
+ "<my:Color>" + color + "</my:Color>"
by value from Infopath form (right-click myFields from Main Data Source > Properties > Details)