Solution
To open a video file in LabVIEW, you will want to use the built in ActiveX control that will open the file in Windows Media Player.
Consult this example that contains a downloadable .zip with a VI that demonstrates that behavior.
If you need to open the video file in a SubVI, you should call the VI by reference to prevent ActiveX from loading the container before the desired time.
Since you will be playing a video in the SubVI, it is likely you will want to open the front panel of the SubVI. This can be accomplished programmatically to allow you to control when the front panel of your SubVI opens and closes.