Best starting cap of vasari is Migrator hands down.
There is no discussion weather you might want kortul or marauder at start.
None, period.
Only time you might consider going marauder is when you are having clanmatch with team that know wtf they are doing.
The Kortul and Desolator siege cap you can leave untouched. They are useless.
Wether to build or not to build second cap is largely desidet by the factors of following:
a) How far away is your opponent?

Was your starttig location good on eco side?
c) Do you find lots of neutrals you can bummrush with fucktonn of scouts?
d) What will you benefit from othr cap?
The "d" is realy important point. Cap ship is expensive pice of space driftwood in hand of unskilled player. Low level cap lacs firepower aswell as lasting power. And is easy to conentrate at. Capital ships are mainly
tactical advantage, only migrator becomes Strategick wepon and thats at level 6.
You need lots of skills and gimmicks to make the second cap useful at lower levels.
And remeber, for the price of 1 exstra cap + research, opponent can get enought ships to take it down roughtly in 30 seconds, give it or take 10.
And when you play the "good players", you know, the one that fucks you up after 15 minutes, you might end regrettick you got the cap.
Worst case scenario whuld be you getting bummsekcsed by fleet of 25 illuminators + progenitor, while you got 2 caps and odd 10 assilants.
Now, one might ask: Why to build 2 or 3 caps at all?
I will tell you.
If you are confident in your skills, have steady income from lots of neutrals in first 15 minutes of the game, you might want to go tad bit eco.
Going eco means you might want to leave your fleet support at second upgrade.
This is where capitals come in!
Properly used capitals can make your small fleet enought to take on 30% to 50% bigger than you, and make you lose less of your ships.
When you rely on 3 capitals and 30 assilants (or maybe 15 carriers, depends) you want tactical advantages.
The combination of Jarrusal > marauder > Carrier is my mostly used capitalship combination.
Jarrusal is responsible for 2 things, gravity bombing, and capitalship gankker. In big fleets you want to leave the nano thing alone and save antimatter for gravity.
With it you have always an option to kite your opponent and get out of harms way.
Marauder is responsible for 2 things too. It can effctifly screw up your enemies attack queue, save ships that are focused stop them from phasing. It can also help your fleet to kite the enemy, realy useful on carrier heavy fleets.
Carrier cap is responsibe in keeping your stuff healed. It is best regen healer vasari got.
Also, litle known fact is that: Repair cloud and regenbay stack, you will get the regenbay + the cloud regen. Use it to pump stupid amaunt of health in to capitals. Best theoretical healing achived is TEC upgradet bays + lvl3 cloud, 55 Hp/sec

Also, note, that repair cloud continue repair ships that are phased out, so before you phase out that woundet capital of yours, hit the repair cloud and
only then phase out hull. That way you get free HP, rincne and repeat and you can make enemy waste his DPS.
All that above needs lots of micro and tactical skills and your day can be ruined by single radiance. Remeber your weakneses, and you can beat fleets while eco whoring