Which stat (VIT or STR) you should focus on as a tank depends on who you are talking to. With the upcoming changes announced for 3.2 it seems like VIT will be the best choice however.
At the moment you want "enough" VIT to survive and then STR for extra damage. What constitutes "enough" is ill-defined.

Regardless of which tank class/job you end up doing it will be wise to get gladiator up to level 22 at least for the Provoke ability. That is a very useful cross-class skill to have when tanking.

It will take a while before you can switch to Dark Knight. First you will need to hit level 50 and complete all the level 50 Story quests - and there are quite a few of those.

Marauder/Warrior might well be the best choice if your goal is to become a Dark Knight - if only because then you will not need to do any levelling of Conjurer which is a requirement for Paladin, but not for Warrior or Dark Knight.
But I really don't think it matters much if you have Marauder/Warrior or Gladiator/Paladin as your main class up until then.