SCVMM R2: Is there possibility to make P2V conversion on working Windows and synchronize changes made during conversion after whole process

As I understand BITS - it's a sort of low-priority service. If other network traffic on that machine (presumably, the existing VMs) are saturating that bandwidth, BITS would (working as intended) never get a chance.

Do you have the luxury of suspending the other VMs to see if the timeouts persist?

Code 39 in Device Manager generally means that there's some function that the driver is trying to invoke which doesn't exist in the NT kernel. (Unless the images is in fact corrupt, which probably isn't the case here.) In this situation, that probably implies that VMBus is trying to use something that is missing. This could happen if your guest OS doesn't have all the service packs installed. If I remember correctly, Vista needed at least one patch to be supported.

In order to install the patches, you can either mount your VHD file and use the Vista offline patching mechanism or you can install a "legacy network adapter" in the VM and download the patches from Windows Update.