I believe the problem is fairly simple.
You have an interface up, and establish a VNC connection across it. Then you try to make this interface part of a bridge. The first thing that happens is that the interface goes down. At this point, any connection across the interface is terminated. Then the bridge is formed, and brought up. At this point, the bridge may or may not have the same address as the original interface, but the connection that was there is already gone.
The secret: configure the bridge in
/etc/network/interfaces, and let the boot process bring it up.
A related secret: Run a DHCP server somewhere (the PI can do this well, as can most internet routers) to assign private use addresses to all the machines.