Manish,
Could you please provide a few more details regarding your problem. First, I am unfamiliar with VB++. Are you using VC++, VB 6, or VB .NET? Perhaps, I am just unfamiliar with the language that you are using. Second, do you have a problem when you try to call the DLL from LabVIEW, or are you just having a problem when calling the DLL from your VB++? If it is working correctly in LabVIEW, but not from VB, then you may have better luck posting on a general VB forum or their may be a forum regarding your specific application. Normally, problems with DLL are related to type libraries not getting loaded properly because of missing dependency files. You may try checking out programs such as Dependency Walker to check your DLL and ensure that all support DLLs exist on your system.
Thanks,
Tyler T.