I personally would use wordpress or joomla . If you want to have a smooth integration of users i would use joomla as it has good plugins for integrating users. It does have a couple good shop scripts also. I personally used wordpress for mine as i did not need user intergration and shop. If you want to take a look you can. Here is my site ..it is digital downloads also.
www.cg-hq.com
also i have done alot of sites complete vb and it never works out for my clients. VB is not verse enough and vbadvance is not up to par yet. Any questions let me know ..also if need to chat about either things just PM me .I have done tons of forums and always used different front end ...I go by that old saying " Don't put all your eggs in one basket "
also integration of virtue mart not sure what you are wanting ? I do not like integrating my shop cause then people have to sign up on your forum and sometimes you get guest that just want to buy ... just depends on what your niche is i guess.