It is possible to have multiple VSI selection steps in a VSM; however, I am not certain why you could not have a single VSI handle the selection of different responses.
Can you describe more about the assertions and why this leads to the need for multiple VSIs?
What are you asserting on to determine which VSI to send the request to?
If the properties that are being asserted exist in request argument list, the VSI should be able to handle the selection for you by examining the argument and utilizing the 'comparison operator' and 'value' assignments.

Using the above as an example, the VSI have different responses based on:
If color = "blue", send response 1
if color starts with the letter "g" (using Regex), send response 2
if color is red or yellow or purple (using Regex), send response 3
META sends an error response indicating that none of the above specific transactions were matched.
There is also another approach called a Match Script that provides the ability to add your own custom comparison logic.