ThumbnailImage method allows a user to specify only width, but it does not work correctly for portrait images (or it is not correctly documented)
For example, if you write the following code

var image = Image.NewFromFile(filename);
Image thumbnail = image.ThumbnailImage(300);

for the image with dimensions 600x900 then it would produce 200x300 thumbnail instead of 300x450.
It appears that it translates to vipsthumbnail.exe -s 300 instead to vipsthumbnail.exe -s 300x

A workaround is to calculate height or to give some large value for height
Image thumbnail = image.ThumbnailImage(300, height: a big enough number)