**Note 1**: In the comments OP requested examples of syntax files in both formats, so this is not a complete answer but maybe some elements which could be useful to have in mind.

**Note 2**: My knowledge of Notepad++ is pretty limited so as said in the Note1 let's hope that someone with a deeper knowledge will see this post and make a better answer.

First to answer the question, as said [here](https://superuser.com/a/575529/386795) it seems that there is no existing tool able to automatically convert a Notepad++ language file to Vim syntax file. The main reason is that the vim syntax files use some very specific mechanisms which don't have equivalents in Notepad++. Hence translating a file might be much longer than writing one from scratch.

Here are some resources which might be useful:

 - [A documentation](http://docs.notepad-plus-plus.org/index.php/User_Defined_Languages) about user-defined Notepad++ language files.
 - [A list](http://docs.notepad-plus-plus.org/index.php?title=User_Defined_Language_Files) of Notepad++ language files.
 - [A wikia tip](http://vim.wikia.com/wiki/Creating_your_own_syntax_files) and [a really good tutorial](http://docs.notepad-plus-plus.org/index.php?title=User_Defined_Language_Files) about Vim syntax files.

I think it is possible to get the keywords on the Notepad++ file to create a basic Vim file.

Once again this answer is not complete at all and really doesn't solve the problem.