Because not use this elementary tag of basic HTML :
<link rel="stylesheet" type="text/css" href="css/myvendor.css">
The path that have to include the tag if must be of the server or client.
If the folder vendor contain all file html and folder that contain file that you using for site, the path that I include in the tag before is correct.
If still not include this file css , the problem is the file not exist or declared wrong file name or structure css's wrong (Is better that you publish file css) but I think that you find the problem with Inspect Element or Firebug (add-on for firefox and [perhaps] Chrome)
UPDATE: Ahhh if you using Jade ,adds 'type='text/css'
inside the you tag because is better that you define the type of file.
link(rel='stylesheet', href='css/myvendor.css', type='text/css' )
Reading the documentation of JADE and on
Template inheritance
If you have multiple views with a similar structure, you will end up with the same set of includes in all your views. This approach can quickly become complicated and untidy, if the number of views and includes increases.
With Jade you can generate the tag including file css with:
!!! 5
html
head
title Learning Inheritance
link(rel='stylesheet', href='/css/myvendor.css')
body
!!!
Or there is a other solution posted in other question on StavkoverFlow
head
title test include
| <style type='text/css'>
include css/myvendor.css
| </style>