Hi Abhishek,
If your VSL fails then the standby switch can’t tell if the active switch is still there or not. If the standby switch goes active while the active switch is still there, both will be active, forwarding packets and you’ll run into issues. This is called “dual active”.
Dual-active detection is configured outside of the VSL link. There’s enhanced PAgp, BFD and Dual-Active Fast Hello Packets to use. You can use one or all three at the same time.
Cisco has a pretty good document for this:
Rene