Hi,
Check KnowledgeBase 2ILD4BXH "SCXI and DAQ Channel Scanning".
You can find it going to:
ni.com -> Support -> Search for: +virtual +channel +numbered
In the KB you'll find general knowledge regarding the use of virtual channels as well as the specific case about mixing virtual channels and numbered (physical) channels.
I hope this helps,
Alejandro Asenjo
National Instruments
Applications Engineer