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 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:
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.