Why does vspace add vertical space after the first line of a paragraph?

Question

I have defined a custom text font command using :
DeclareTextFontCommand{desc}{
    fontsize{11pt}{11pt}
    fontseries{light}selectfont
    vspace{10px}
}

And I have used it like that :
desc{Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed at tempor ex. Praesent a luctus enim. Nam mauris odio, ornare at egestas ut, mollis aliquet est. Nunc a rutrum tortor. Suspendisse et odio rutrum, porta ipsum non, fermentum turpis.}

I expected to have a 10 px space after the paragraph, but the space is added after the first line :

Also, why are the second and third lines are not aligned with the first line since I removed the indent of the paragraph? Those are shifted left in the margin of the page.

Don Hosek · Answer

vspace adds the space at the point where it's called. So, for example if you write
Foo

vspace{1in} 
Foo

The space goes betwen paragraphs, while
Foo
vspace{1in}    
Foo

puts it after the line on which the first Foo appears. So surely, we should see, if we have
Foo

desc{Foo}

that the space comes between the paragraphs, right? Except that you're using DeclareTextFontCommand which does a little bit more. In particular, it does:
def DeclareTextFontCommand #1#2{%
  DeclareRobustCommand#1[1]{%
    ifmmode
      nfss@text{#2##1}%
    else
      hmode@bgroup % ❶
       text@command{##1}%
       #2check@icl ##1check@icr
       expandafter
egroup fi
}

❶ is the critical line here. hmode@bgroup leaves vertical mode (so it begins the paragraph) and then begins a group. So your vspace is being output after the paragraph begins and comes after the first line.
Most likely, what you want is not to use DeclareTextFontCommand at all. Assuming that you want to use this as a marker at the beginning of paragraphs, you could instead do:
NewDocumentCommand{ m }{%
   par
   vspace{10px}
   {%
      fontsize{11pt}{11pt}%
      fontseries{light}selectfont
      #1
   }%   
}

to get your desired effect.
(Side note, I'd not seen px as a unit in TeX before. It's apparently equivalent to bp or the PostScript point of 1/72 in. It's not mentioned in The TeXbook as far as I know. Apparently an eTeX enhancement as it's an illegal unit if I use tex but is available with pdftex and any of the LaTeX engines which all require eTeX.)

Olon · Answer

Environments are easier to use and and fixed my problem. The second {} contains the styles to apply before the text and the third {} contains the styles to apply after the text.
newenvironment{desc}{%
    fontsize{11pt}{11pt}%
    fontseries{light}selectfont%
    begin{spacing}{1.1}%
}{end{spacing}vspace{10px}}

Adding % after each line fixed my second problem. Without these, whitespace would shift the text right.

Why does vspace add vertical space after the first line of a paragraph?

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