You are getting a return value of 1 - which means an error of some sort.
subprocess.check_call so you get an exception raised if the command fails, you'll get a bit more about why it fails if you do this.
A couple of possible problems:
You are not passing the filepaths, you are passing the literal strings
outVRT (note lack of quotes) instead.
Your subprocess call will also fail if
gdalbuildvrt isn't located in one of the directories in your
PATH environment variable. If that is the reason your command is failing, either pass the full path to gdalbuildvrt, or use os.environ to append the gdalbuildvrt directory to your
subprocess.check_call([r"C:\Path\to\gdalbuildvrt", "-separate", "-input_file_list", inlist, outVRT])
os.environ['PATH'] = ';'.join(r'c:\Path\to\gdal\bin\directory',os.environ['PATH'])
subprocess.check_call([r"gdalbuildvrt", "-separate", "-input_file_list", inlist, outVRT])