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There are some differences. Besides those that Philip already noted, VSS is limited to two members, while Cisco stacking switches can usually include up to nine members. Optimal design, for VSS, traffic should not cross between VSS members, except in failure situations, where stacks use their ring links somewhat like an internal chassis bus.
In answer to your question about what happens if you lose the VSL link, well if you loose all the dedicated connections between the two VSS members, the two VSS member will partition themselves believing their partner has failed. This is somewhat similar if all stack links are lost between some stack members, where the stack will partition itself, each member, or still joined stack members, consider themselves the stack.
