Apparently this is NOT supported and they'll only add it if enough people suggests a feature... which I don't understand because Vegas Pro is supposed to be for broadcast but it can't export files suitable for broadcast with networks specifically needing 720p60 .mxf and as a general rule... NEED DROP FRAME TIMECODE.
Here's my post from the other day:
http://www.sonycreativesoftware.com/forums/showmessage.asp?messageid=926938
Post suggestion here:
http://www.sonycreativesoftware.com/support/suggestions
Here's the subject I used: DROP Frame Timecode in Sony MXF > HD422 1280x720-60p 50 Mbps render template.
Thanks!