However whatever works for the VSD should work even better for an ISD.
As a general rule, that is correct. However, it should be noted that ISDs are roughly 40-50% more expensive than a VSD of the same number (ISD-I vs. VSD-I, etc.). This gap begins to close when you put the same upgrade cards on them, however.
Example:
A naked ISD-II is 120 points
A naked VSD-II is 85 points
The ISD is 41% more expensive than the VSD
An ISD-II (using only upgrades a VSD-II could take, for purposes of this example) w/ Gunnery Team, XI7 Turbolasers, Intel Officer, and Leading Shots is (120+7+6+7+4) 144 points.
A VSD-II using the same upgrades is (85+7+6+7+4) 109 points
The ISD is 32% more expensive than the VSD
You can make the argument that the naked versions are equivalently priced given what you get but once enough upgrades get involved it becomes clear that springing for the extra 32% (as in the example above) is a much better use of your points. Hence my basic argument that the less upgrades you're putting on your VSDs, the better. It allows them to do their thing most cost-effectively. In a competition versus moderate-to-heavily upgraded ISDs, it becomes more difficult to justify their cost.