I am having a problem with getting an empty cart randomly when adding items.
At first it appeared to be a problem with adding too many items, but after more testing it appears to be random.
http://bit.ly/1NIk8vz
VM2.0.6 is 4 years old - seriously out of date and insecure. http://forum.virtuemart.net/index.php?topic=118683.0
It is possible that updating will solve the problem you see (assuming that the site has not been hacked), but you will very likely need to rework your template overrides. There were problems with ordering too many products in older VM2 versions but that was later fixed.