Vasari repair bays still have oh what is it? 20/25* hps for 10 seconds I believe. Now, that may not seem like a whole lot, but that negates the damage of multiple illuminators all firing on one ship. I'd say its a pretty good deal. That and unless I am mistaken, Vasari repair bays have the shortest cooldown. Sure, they don't have the massive 40 hps of TEC (which require research to be oh so good btw), but its still good.
So, I do believe that doing so would make your ships last twice as long. Yes the Advent are OP, but in the defender always has advantages. The largest would be repair bays, and if you don't use what is available to you and then blame it on the OPness of the Advent, all you are doing is kinda-half-trying to win.
Let's look at some comparisons:
Illuminator=Assailant (I say this because Assailants come out earlier and have PM's which become awesome late game)
Defense=Sentinel (see previous reason)
Now, you know you will never get anywhere with Navigators and Skirmishers. Its common sense. They cost more and do less. They are not worth it in the least. For this reason, you know you could never win that way. So what options are left? Assailants, Sentinels, Capitals, and Repair Platforms.
Assailants are going to ripped apart so they are out as the key to victory. Sentinels will get shredded by Disciples, so they are out. Capitals are going to draw a blank, though they will help level the playing field. The only option left is your best. Repair Platforms. While they do no direct damage, they heal you at 20/25 HPS for 10 seconds. That is 200/250 hull points. And I believe that most frigates have 3 armor and mitigation goes up to about 60. Let's see what that means.
Damage taken is more or less this formula:
However, healing while taking damage is the following...
So, let's switch up the formula to find how much damage it takes to punch through it, shall we?
And let's say that the repair platform heals 20 HPS. You then simply solve for DPS.
So, that means that a single repair platform can hold off at least 6 Illuminators against one frigate. The result is higher if used on a capital. That means that while you may be outgunned, your ships are going to last a whole lot longer than theirs will. For that reason you ought to be able to hold them off until you become advanced enough to counter. Late game, the Vasari can slaughter forces with all their toys. Sub+Ruiners for instance is a very deadly combo. I used that against a hard AI the other day for the first time..
A fleet of about 1000 FC went away. Capitals died, frigates died, everything went BOOOOOOOM!!!! After seeing it, I didn't know whether to be terrified or happy, so my result was nervous laughter. I checked the stats as well. Vasari Emines are by far the most devastating as they have a huge range and then of course deal massive damage.
Back on subject... You have to hold out and colonize, building up your military and economy before slamming back against them with everything you've got. I know Vasari can't win early game, but if you can hold them off (which you should be able to) then you can bring out the Vasari's toys, and at that point, you can win.
*I'm not sure which it is, so when I did estimates, I was conservative. If it is 25 HPS, then you end up with 70 DPS neutralized or nearly 8 illuminators.