Hello Sathish
In the context of MST, the “Virtual Bridge” is a virtual entity that the MST region creates to interface with STP topologies that are outside of the MST domain. This virtual bridge is seen by the switches outside of the MST domain as a bridge with a specific bridge ID. So from the point of view of switches outside of the MST domain, they communicate and exchange BPDUs with that virtual bridge as if it is a single physical device.
The virtual bridge however is actually not a specific physical switch. It is actually a construct that is created within the MST domain using the process that Andrew described. The best bridge ID is determined from all the switches in instance 0 of the MST domain, and that bridge ID is shared with switches outside of the MST domain.
The result is that any switch outside of the MST domain that has a connection to one of the MST switches will see the whole MST domain as a single switch with the bridge ID that has been determined by MST as the best bridge ID in the domain. In this sense, switches outside of MST will perceive the whole MST domain as a single switch with the bridge ID that MST determines to be the best in its topology. Does that make sense?
I hope this has been helpful!
Laz