Vista - Issues with bridging between VPC and host OS network adapters

First let me explain the configuration I use:

I have Windows 2003 Server R2 running on Quad Core Xeon and 4GB of RAM. I
have only one network adapter - Marvell Yukon 88E8056, latest Win2003 driver
version -10.64.2.3.

What I want to achieve:

I want to have access to multiple private customer's networks from this
single machine. Since the network adapter supports 802.1q (VLAN tagging), I
can configure the port of the switch I'm connected to as 802.1q trunk, put
whatever VLAN I need on it and create virtual network adapters in Windows,
each one associated with a certain VLAN ID. With this scenario everything
works fine - I can have access to multiple networks in different VLANs and
each one is in different subnet. So far so good - still nothing related to MS
Virtual PC The problem arises when I want to access overlapping customer
networks, beyond the subnet of a given VLAN. If I need access to network
192.168.1.0/24 on Customer #1 behind virtual network adapter #1 or to the
same subnet 192.168.1.0/24 on Customer #2 behind virtual network adapter #2
there is no way the OS can distinguish which network adapter to route the
packets to. So, I've decided to use MS Virtual PC and create a VPC for each
customer. Then I bridge each VPC network adapter to a specific virtual
adapter (with specific VLAN ID) on the host OS.
Now, in theory, I can access overlapping subnets since each customer VLAN is
directly associated/bridged via the host virtual adapter to a given VPC.

What's the issue:

I have three customer networks => three virtual network adapters on the host
OS => three VPCs. When the bridging on a given VPC is configured with one of
the host's virtual adapters, all communication through all host virtual
adapters stops! Let's say none of the three VPC is powered up and there is a
separate continuous ping issued from the guest OS through all three virtual
adapters - and it works fine, I'm pinging the default gateway of each subnet
behind each virtual adapter.
I power up one VPC and as soon as the OS boots up and the VPC network
adapter (bridged to virtual adapter on the host) comes alive - all ping stops
trough all virtual adapters on the host - and there is no ping between the
guest OS and the host OS or to any IP form the certain VLAN. I shut down the
VPC or just disable its network adapter - and the ping goes again!
Any ideas what might be the problem? Some kind of spanning-tree protocol on
Windows?