t. Is a DDL for a PIC32 to connect to a PC via USB different from the PIC18 DLL?
No, DLL only depends on the firmware functionality. As long as you use a compatible firmware there is no need to change any programming on a PC. This website is committed to provide a compatible DLL for all supported microcontrollers. The only limiting factor is a microcontroller firmware version. Higher versions support more functionality that is made available by the SV LIB DLL.
Please, refer to the programming guides for details:
PROGRAMMING GUIDE: How to start programming PIC18F2xJ50?
PROGRAMMING GUIDE: PIC32 microcontroller programing in VB.NET