You can get very usefull information on the Java Tutorial, take a look at the JNI chapter.
Basically, what you'll have to do is (from the tutorial):
1.Begin by writing the Java program. Create a Java class that declares the native method; this class contains the declaration or
signature for the native method. It also includes a main method which calls the native method.
2.Compile the Java class that declares the native method and the main method.
3.Generate a header file for the native method using javah with the native interface flag -jni. Once you've generated the header
file you have the formal signature for your native method.
4.Write the implementation of the native method in the programming language of your choice, such as C or C++.
5.Compile the header and implementation files into a shared library file.
6.Run the Java program.
As your DLL is from VB, I gess you will have to write a C DLL as the native method implementation, which in turn calls the VB DLL. Not sure 'cause I do not know VB, but I suppose you cannot use C headers on VB.