Here is what I would try:
a) Write a simple Java command line app that used JNA to load the vix.dll
b) Use Process Monitor from sysinternals to see which what files ar actually being access.
c) Get the source code for JNA and see if there is some detailed logging available to debug loading issues. If so, turn it on. If not, add logging code to it.
Basically, you have to debug JNA and find out why it is not working correctly.