Send data from LabVIEW to VC is "inter-process communication" or IPC.
You can find many solution by searching on internet.
The easy ones, such as write to a text file, write to clipboard. etc.
Advanced method, for example: send window message.
The right way to make a dll / exe to show up in LabVIEW is to
change the dll / exe to ActiveX or .net if possible.
Otherwise, you have to make the 3rd party window a child window of LabVIEW.