Hi,
If you want to make calls from LV to Delphi than I would create a dll of my Delphi application and import the dll in LV. If you want to call LV code from Delphi than you can easily create a dll of your LV code and import it into Delphi.
So a DLL is the best way to interface between the two programs.
Another way is to use activeX. Check the link below:
http://sine.ni.com/apps/we/niepd_web_display.display_epd4?p_guid=B45EACE3E8D156A4E034080020E74861&p_node=DZ52051
Regards.
JV