I can see that as well. 7 points may not seem like a lot but it can be huge when it comes down to it. Hmmmm at 400 points I can fit 3 VSD's (1 or 2's) and 2 GSD 1's so yea compared to that I am not sure how good 3 ISD's will be.
Though that list has no upgrades but Skreed. I think I would use the VSD 1's so I can add upgrades to the GSD's.
I've tried one of those 3VSD/2GSD fleets. They're pretty stripped down--ACMs on both GSD Is, plus one Demolisher title leaves room for 3 VSD 1s, Motti/Screed, and 21/19 left over for VSD upgrades and/or your bid. You don't have much margin for error when you are engaged, but 3 VSD front arcs can deny an incredible amount of space on the board. Run maneuver commands for the first two or three turns and recklessly bull-rush at speed 2, and ACMs are almost viable on the VSD (expanded launchers maybe, too, though the price is a little high for my tastes; either way, odds are someone's forced into close range, either by the Gladiators or one of the other Victories pushing forwards). If your objective cuts the board size down (minefields, fleet ambush) or forces both sides into one spot (contested outpost), even better.
Plus the look on your opponent's face when you start is priceless...
Yet, my squadrons can easily kill 2-3 ships alone in a 6-turn game...especially if SupPos or precision strike gets picked, and this type of list has no answers to that. 5 50/50 rolls and 3 black dice is going to shred people who can't engage them.
Furthermore, if I'm comfortable tangoing with 2 impstars, 2 Vic's will not even remotely phase me. Plus, I'll have a raider with a ton of toys to combo with my demolisher to delete a flank.
I am not sure how your squadrons will kill 2 to 3 ships in a game. They will take shields down but unless you are killing nebs and CR90's I think you will get at best 1 maybe 2 if you roll REALLY good